Does prop slip increase or decrease with speed? Top speed less than expected.
 

I ran my boat Sunday for what I was planning to be the final run for the season and finally had a chance to really crack on it before it went on the trailer. Seems like I always had a crapload of people on board every other time I had it out this year, so I went easy on it for most of the season. Top speed was considerably less than anticipated. I plugged in some numbers into the go-fast.com prop slip calculator, and I got the following slip calculations:

I am running a 1.36 Bravo 1 with a Mirage Plus 23. Boat is a 1988 Baja Force235 with a 489 running AFR heads, Holley 800, Lightnings, etc. This build has been pretty well documented on other threads here, so most of us are familiar with it. I had the engine dynoed and saw 585 HP at 5400 RPM and between 575-600 ft lbs of torque all the way from 3000 RPM up to peak HP.

At 3000 RPM I was seeing about 39 MPH on GPS. Theoretical MPH at this RPM is 49, so I have a slip factor of around 19% at this speed. Fair enough, there is a lot of hull in the water at this speed.

At 3500 RPM speed was showing about 48 MPH on the GPS. That equates to a slip factor of 14%, which is pretty much as expected.

I ran it on up to WOT, and was only able to make it to 61 MPH. RPM was only 4600, and that comes out to a slip factor of 18%. Something seems amiss here, both from the higher slip number and the fact that I wasn't able to spin this thing faster. I played around with trim and tabs, but did not seem to make a difference on top speed. Why would my slip number go up, unless I had it over trimmed?

It seems to be making plenty of power at lower speeds, getting up to 50 MPH is pretty effortless, so engine seems to be running OK. Did not note any miss or notice that it seemed to be running poorly, but it just wasn't bringing the power. Dyno runs were made with dry dyno headers, and A/F numbers were around 13 from 4000-5000 RPM, so I would expect it to run a little richer with the wet exhaust and muffler tips on it. I have yet to check the easy stuff like throttle opening all the way (it was with the other engine, and nothing changed in this regard), timing, etc. I was running speeds like this with a 454 that made less than 400 HP, so I would have to be dropping at least 100 HP if it is an issue with the engine.

Anyone want to take a stab at it? Maybe my boat is just a pig! :p

Bud are you sure you are running on all eight holes? With almost 600hp I would think that little mirage p-shooter would be annihilated especially with that kind of slip. I would pull the plugs and see what they look like. I've said this b4 but been argued the 3blade is not the way to go. Make sure you don't have a run issue then worry about the slip.

That's a lot of horsepower, but a lot of gear too. I am sure there is a reason but why 1.36? Try a 21 maybe get you into your power band, if not was the dyno really happy?

Black, pretty sure I'm firing on all 8, but I will pull plugs just to check. I guess I could be having some other kind of ignition issue to knock the power down too. Like I said, I haven't really started troubleshooting it yet, so this post may have been premature.

Buddy of mine has virtually the same boat (89 Baja Force) and tried a 4-blade Bravo 1 Prop and it did not like it. Geve him too much stern lift, when what these hulls really seem to need is bow lift. The Force models have the helm pushed up further than most go-fast hulls, so there is probably more weight centered towards the bow. Mine seems to run with the bow pretty well planted most of the time.

The 1.36 is what the boat came with, has been on there since I bought it in 99. A few years ago I swapped my 1.36 / 21 Mirage for a buddy's 1.5 / 23 Mirage Plus and saw virtually no difference in performance.

Maybe the dyno was a little "happy" - no real way to know for sure. The power numbers were in line with what Bob M expected with the cam and AFR heads that are being run - they weren't out of line for similar builds. Even if I'm not making the advertised power, it would have to be "happy" to the tune of at least 150 HP for these speeds.

Come to think of it, my 12 year old was riding up in the cuddy at the time, and I had a ski tube and some other junk in there. However, it's hard to imagine that this could account for 10 MPH. Something else is up here.

Are you at the limit of the hull/drive combo? Like maybe time for a shorter lower?

You should be able to put the fat lady in the cabin of that boat and still scream along pretty good if you truly have over 575 ft lbs of torque from 3000-5400 rpms. How is the bottom of the boat ....Does the boat seem to free up at speed ..Or plough water ...Something doesn't add up ...Who dynoed engine..........?

OK, sounds like it's more involved, was just thinking out loud.

Where do you run the boat?

I'm having the same issue on my Baja 272. Fresh 500+ horsepower, tried different props, but still running 23% slip @ 54mph in a boat that does over 60 with the stock 454. I'll be following this saga closely.

Cole, my buddy Tim was running over 80 MPH with essentially the same hull / drive depth combo. But maybe it's just something about my boat. Checked hull for straightness in the past, and actually had factory "hooks" removed several years ago.

motor, engine was dyno'ed by Dale Meers Racing Engines near Hodgenville, KY. Stuska dyno running Performance Trends DataMite data acquisition software. He does primarily drag, circle track and truck pull stuff, but has a pretty good reputation around these parts, especially for the MOPAR guys.

This run was on the Ohio River, north of Westport KY.

Did you dyno yours before running it in the boat?

I'm going to give the engine a good looking over, basic health check. Only has about 20 hours on it. Wish there was a way to run an O2 wideband with these lightning headers, but as others can attest, they love to kill sensors.

Budman II, Sorry to hear about your performance after all the hard work.
I would think it should go faster also. The slip factor doesn't seem that terrible, mine runs at 17-18% slip, but it's a bigger boat. A stock 502 should get you to 60mph, so it seems that you are loosing power somewhere. I would definitely start with the easy stuff first, like timing and throttle. Also a dead cylinder/broken spark plug etc. could cause the issue and you probably wouldn't hear it. Try it again after you get most of the weight out of the front. I can take 10 mph off mine with extra weight in the bow. Something seems to be holding it back. I know it isn't a real fast hull, but at 60 your not asking that much out of it.

I think you have to much prop. We put a 496 in an '89 Baja 240 and it runs 72 with a 23" 3 blade or a 22" Bravo1 and 1.5 gears.

Too bad you weren't close by. I would love to become involved with your puzzling man against machine war. My problem is I don't give up early enough. LOL.

heh, first I would do , if not already, is check with other 'same exact boat' owners and see what prop works best. I'm not necessarily talking just pitch here.


I think a Prop dyno would be great here. Find out if there is some tuning or rigging issues going on.

Example: My insert mufflers cost me a little over 2mph.

Example #2: My 9x4 K&N flame arrestor costs me a little over 2mph. My big bad testing assault day it was closer to 3mph.

Disclaimer: Remember, my current little (20ft) sbc boat responds + and - mph wise to anything and everything. I swear I can change mph by turning my hat the oppsoite direction. LOL.

Also, to answer the original question, I have always got the least prop slip at top speed with slip increasing down though the midrange.

The sensor itself is an $80 part mass produced by Bosch for Volkswagen originally.

Prop Dyno would be ideal. Trying a smaller pitch is worth a shot too. You're propped to run up to 80+ at 6000rpm.

Something sounds way off to me. Nearly 600hp and only 60mph in that hull? I had a 241 Liberator that was prob quite a bit heavier and larger boat than a 235 Baja. That boat would run 70mph with 590hp single and a deep bravo.

Are you sure she's getting enough fuel at wot? AFRs of 13 is too lean for both safety and power.

What is your carbs list number and what jets in there ?

MT, yeah, a lean condition (or rich condition) is in the back of my mind, and I agree that there is almost no way I was making the dyno numbers to run those speeds.

In regards to jetting, I don't have the list number in front of me, but it is a Holley 800 straight off of an HP500. Jetting and dyno numbers are detailed in my dyno thread:

http://www.offshoreonly.com/forums/g...o-headers.html

From the thread:

"We started with the stock jetting that Holley puts in the list# for that carb - 72 primary and 87 secondary, with a 6.5 PV. First run was with this setup and 32* total timing. Went easy on it and only took it to 4700 RPM, and we got 484 HP and 540 ft/lbs. Next run we richened it up a little, and got 530 HP at 5400 RPM, similar torque numbers. We eventually got to 76 primary and 84 secondary on the jetting, same PV, and 34* total timing. At this point we were at about 565 HP and 580 ft/lbs. We tried an HVM Super Sucker 2"spacer on it, and it netted us a gain of about 10 HP. "

I believe 76 and 84 is where we left it.

In regards to the A/F ratio being too lean at 13+, that is what we saw with dyno headers. I asked about a correction factor when going to wet headers and mufflers in the boat, and got one response from lil red:

"if your using a wideband 02 in the exhaust if look for 13.1-13.3 a/f on the dyno and it should be 12.6-12.8 in the boat when you put your exhaust on etc."

He is the only one who addressed my question on the A/F ratio, and no one disagreed, so that is what I went with. So I am doubtful that I am seeing A/F numbers in the 13's on the water, but you never know. I sure wish there was a way to run a wideband with those Lightnings I have, or I would be all over it.

BTW, that damned avitar you are using is distracting the hell out of me. I always have to read your posts three times to absorb what you are saying because I'm looking over there waiting for her to take that thing the rest of the way off. ;)

I'm running a 21p laser on my 22' velocity. Turning 6000rpms and getting 75 no problem. I don't think a 23 would be to big for you. Is there a possibility you are over trimming the boat? If you have another prop to try I would start there

The problem could also be, in the fuel supply. Like too small a pump, too small a pickup in tank, etc. Is your fuel pressure holding enough fuel psi at wot?

When you guys dyno'ed, did you keep increasing JET until you saw a drop in peak HP? Or simply decide enough was enough because of the AFR readings?

I have found that the AFR readings on the dyno, can be misleading as far as power production. For example, ICDEDPPL's blown 540, made its best number, right at 11.5 AFR. Any leaner, it dropped power, any richer, it dropped a little.

My gut tells me, you had some more power in the engine with some more jet, even though you are N/A. Alot of guys have the mindset that lean means bigger power numbers. You really got to keep giving it what it wants, and let the dyno kinda show you whats up. The AFR is sort of a byproduct (to an extent).

If fuel pressure is holding steady, I think i'd begin throwing some jet at it. Especially since the temps are cooling down now, its not like its super hot humid summer air. Now is the time to dial her in while its good air.

Bud, lets think about this for a minute. You mentioned the carb was off a stock HP500 merc engine. That engine, made 470HP at the propshaft, and probably if you were to dyno one, on the dyno you did yours, it probably would have made around 520HP?

You are running a total jet diameter, of .366 when you add up all 4 jets. The HP500, had a total jet diameter of .488. So, you are running 25% less jet size (which is alot), but yet attempting to make almost 20% more horsepower (which is alot).

Just for conversation sake, take a 520HP, and drop 25% from that number. Thats 375HP. Which is almost dam near the speed you would get from that hull, with a 365HP 454 Mag.

Hardly scientific, just something to chew on though. It takes X amount of fuel, to make X amount of HP. Theres no getting around that.

MT, I haven't had a chance to check fuel pressure at speed. Thinking about mounting a GoPro aimed at it the next time I run it. We did not try to increase jet sizes in search of more power. We did try running more timing, and it didn't seem to like it. I think we went to 34*, then backed it down a little to 32*. My understanding is that the more efficient chambers on the AFR heads shouldn't need as much advance as the old cast iron heads do. I was mainly trying to get a good safe tune to avoid melting anything down - wasn't really interested in the biggest power number possible, but I know what you mean about going up in jet until the engine tells you that it doesn't like it.

I'll check the fuel pressure, and then look at adding some jet. Should I step up the secondaries first, or step up both primaries and secondaries at the same time?

Why can`t you run a 02 sensor in the lightnings? I had a set and ran 02`s for a long time without any issues.

I did not get richer in the boat, I was leaner and had to jet up from 84 to 91
Personally I wouldn`t go WOT on anything I didn`t know the AFRs on.
You could be way lean.


http://33outlaw.zenfolio.com/img/s9/...49236152-4.jpg

Notice the 02 bungs on top.

At this point we don't know the AFR's or even how the plugs look at cruise. Since you are running a Merc HP500 carb, I believe the same intake manifold?

I personally, always feel its best to start tuning being RICH, then work your way down. You don't want to be doing WOT passes repeatedly being lean.

You got 76/84 in those. If I remember right, that carb has different size boosters in the primary, or something like that. Stock jetting spread was 15 sizes. Which is weird, but thats what they came with. If it were my setup, I'd go 80P, and 94 S, and work from there.

Disregard my prior post somewhat. My morning math was off a bit. still was waiting for coffee to brew. The HP500 carb did not have 25% more jet diameter than what you are running. Definitly verify fuel pressure before anything.

First order of business is dialing the engine in. Props, drive height, and things of that nature, come second. I think at this point, the tune up is questionable, so need to verify all that first.

I was going by what others have experienced with their Lightnings. I think it was Endeaver who had a lot of issues with his sensors getting wet, especially at idle. I think his would live for a little while, but they would die a watery death as soon as he went back to idle. And mine have those funky divorced collectors, which complicates things. You may recall that I had to jump through all kinds of hoops with inner pipe extensions to keep mine from reverting. I guess I could try installing a set of bungs and see what happens, but I'm about out of money for this thing. Job front is getting shaky, wife is losing her job in December, so I can't throw much more money at it, unfortunately.

Guess I just live a charmed life when it comes to boats. :rolleyes:

I had a problem on one of my engines 2 summers ago. One day the port engine would only hit 4500rpm and that was it. No stumble bog or hesitation. Just 4500 and that was it. Normally they turn 5500 no problem. Boost was there. Went back to dock, found fuel pump fitting on inlet side of pump loose. Fixed that went back out and reved right back to 5500

What are you running for fuel pump?

Running a Carter black electric pump. => http://www.summitracing.com/parts/cr...FYxDMgodRUMAxA

Actually toying with going back to a mechanical pump due to better reliability - block has the mounting boss and camshaft has the lobe.

So you were sucking in air at the fitting? I'm going to look for stupid stuff like that, including a cracked fuel pickup.

Budman FWIW we put a 533ci in a friend's baja 250 sport this spring, hit a whopping 60 first time out. It was way lean, threw some fuel at it and hit 69mph gps pretty quick. Listen to Mild, the tune is everything.

How many steps up did you have to go with your jetting?

Quick question - if I was running lean enough to cause me to drop 100+ HP off the table, should I have seen any elevation in engine oil temp? Granted, I probably only ran it WFO for a minute or so.

I'm going to check fuel delivery, timing, etc., and probably try jetting it richer just to see what happens. However, here's what I'm having trouble wrapping my head around: If it is jetted lean now, I would expect that it would have been lean on the dyno too, in which case I would think the A/F numbers would have told us, and it would not have made the power it did. How is it suddenly jetted lean now that it is in the boat? Especially with what should be more restrictive wet exhaust with muffled tips. Not doubting any of the excellent suggestions and advice I have had per se, just trying to understand what's going on.

I agree. Lightnings shouldn't be any different. You just have to make sure you don't put them too close to where the water mixes.

Three years ago I warmed up a couple of HP 500's. They were in the 550hp range running fully dressed and wet exhaust on the dyno. We could not jet the carb fat enough. Ended up with 1000cfm pro systems. My guess is you are way too lean.

When you keep adding jet with no response it's because the emulsion aren't sized properly. That's the nice thing with the new carbs they have jets...

Lightning headers could be scavenging better on top. They make good power.

The fuel psi gauge should tell the story.

Personally, i think that pump setup you have sucks. 18psi pump 100gph pump i see? And you are running a regulator down to 7psi i assume?

I know nothing about those pumps, and cant find the info on what it reallt flows at 7psi. My gut tells me to toss it in the lake though lol

If i were in your shoes, i'd invest in a six valve mechanical pump from baker engineering , or teague sells them. Self regulated , 1/2 NPT ports, will support more than enough power, and simple as can be.