353 Speed Question
 

Roughly 300hour 502 Mag MPI's Bravo 1 drives, four bladed SS 28P props.

What GPS speed should I be seeing at WOT and trimmed out?

Thanks

Boxes? Conditions? People? Fuel? AC? Water? A lot of contributing factors to top end.

No boxes, flat conditions, myself (175) boat girl (119) :-), quarter tank, factory AC, water tank full.
Boat new to me, max has been 51.5 GPS. Something's gotta be off.

Def not 50.

Low to mid 70s. 502s are only rated 415hp they are heavy SOBs too. the Merc 500s have 470 and top end is 75-80s on them. 525s have heard 83 in perfect conditions half fuel half water two people tail wind etc etc. whats the fascination with WOT?

Only real way to know is to drop the hammer and find out.

Full throttle, trimmed with the conditions I've stated, I've only seen 51.5 GPS max. Suggestions?

What RPM are the motors running?

67-72 on gps depending on load and conditions and all... you are way off!

TRim it out, bring tabs up.... OR...... fix whats wrong with it....that's a cruise speed not WOT....

Sounds like a timing issue or fuel restriction

If your trimmed out properly boat should do low 70s. Trimmed out should be no tabs and 3-5 on your drives. What about maintenance to fuel filters, water seperators, and plugs? Are you running marine fuel versus ethanol pump gas?

Thanks for the feedback boys. Maybe I have been over trimming out, let me experiment again and I'll report back.

if you were trimming too far out you would have more rpm, or blowing props out making way more rpm...

That combo will rarely hit 70 but should be close (over 65 all the time). You should also spin about 5,000-5,100 rpms. Seems like 28's might be too big for the boat???? Are they 1.5 or 1.65 drives?

I think I may have been trimming out too far, let me take another run on her and share the findings.

i dont think it's your trimming. With my drives all the way in I would still go over 50mph... What RPM were you at? I had a similar issue this summer where the port motor wouldnt want to rev.. I changed fuel filter and was good to go.

Does the boat feel sluggish? Does it sound as if the engine is straining? I would think running WOT and doing 50 the boat just wouldn't feel or sound right. Plus what's it doing out the back? Do you have a lot of spray? Is it calm and flat? Does the boat feel like the bow wants dip into the waves?

I believe they are 1.5's

I'll go along with that. 26 Bravo 1s will wake it up.
Again what RPMs at WOT??

Update.
Took an ocean stroll to Palm Beach tonight. Mid to 3/4 throttle position, trimmed out to 5 max RPM is 4k and 47MPH.
Advance to full WOT and RPM is 4100 max 51MPH max estimated.

The fuel filters on the motors are new, and I've located an red anodized tubular Flow EZY in-line filter on both fuel supply lines made in Ann Arbor MI that I can not verify when they where new/serviced/changed.

Conditions: 1'ers, just myself, full fuel, full water, GPS based speed.

Whatcha boys think, in-line filters the culprit? Bypass them or replace? Are two fuel filters per motor necessary? Had computer on each motor last week to diagnose a needed relay replacement, code cleared and no other codes found.

Start with checking compression and replacing spark plugs, leak down test would even be better...check fuel pressure as well...My guess burnt valves or fouled spark plugs

You said trimmed to 5. Have you indexed your indicator? At what point are the drives level to the bottom of the hull? 502's and no boxes? Twin step?

No boxes, twin step. Just had a local offshore shop tip to stern the boat, I'd be surprised if its not calibrated properly, I trust they would have done that.

It can't be the valves, for one thing that would be just be too expensive of a fix! Haha seriously though, total hours are less than 400, and had valve job done less than 100 hours ago.

My money says the ancient inline filters are restricting fuel flow.

Index your drives. With a ts you should only be a tick or two above level. Your fuel/water separators should he changed every season. 28's might be a lil big for your boat but should still spin to 4500. I believe they came with 26's from the factory with your setup.

Ok i have the same boat, 353, 502's all stock, 440 hours on them etc....

At 4k rpm and the boat trimmed to 4-5 im at about 57-58 mph or so. ive seen 69.8 on this boat at about 5k and I shut it down, but day in and day out its a 67 mph boat with 3/4 tank 4 people cooler etc....

They came factory with 28's. Your issue is not the trimming.. you are way down on power somewhere.

So what did you come up with?

Both engines are same rpm?

I by passed the inline fuel filters. And still my max RPM is 4000-41000 with same max speed.

Suggestions?

Are your props the original 28s? Any history of props being worked? Could the props be from another boat or props were worked and now too much for the boat? Someone mentioned drive ratio earlier, your 100% sure 1.5s are on there now? Just trying to help, it is a clue that both engines are turning the same low rpm.

This issue with your boat doesn't seem to be a problem that is inherent to only Formula boats. I would post this same question on the Technical "General Q & A" thread, along with an explanation of what you've already tried doing to to resolve it. There are some extremely knowlegeable guys that frequent that thread (some are professional marine mechanics) who might be able to give you additional advice as to what might be going on here... Hope you can figure it out soon!

Do they spin 5,000 rpms in neutral? If so, then it's likely a prop problem. If 4,100 is all they do then you know the problem is in the engine compartment.

Between here and the performance shop I work with, it's starting to become a consensus that I'm over propped. Going to try a set of 24s or 26s.

To the question above, yes in natural it will rev to 5000rpm.

Weren't 28's the stock props on that boat? I think it's time for a compression test and leak down.

I'm going to disagree with you on this one. What a motor does in neutral and what a motor does under load is two completely different things!

Like i said to the previous post.. what a motor does in neutral and what it does under load are two different things.

Your issue is NOT the prop size! Could your prop be damaged? sure. But it is not the size. These boats came with 28's and like I said i have the same exact boat. You are down on power... big time.

It's suspect to me that both motors would have compression problems uniformly.

Same hours on both, that sh!it silent thunder system, and or bad manifolds. I've seen it before.

Leak down test!!! Will tell all

502s with 28P at around 4400rpm I get almost 70mph with a normal load.

A engine that isn't running correct may still spin 5g with out a load. A compression test and a leak down will tell a huge story as to what is actually going on. This is a inexpensive test and will save a lot of time and money vs. trying different props and may save from further damage to cylinder walls if you are having reversion due bad exhaust or internal leaking.

Being that you run in salt water It is very possible that you have internal leaking due to exhaust issues or possible head gasket. Witch leads to low compression either thru the heads/valves or cylinders. Just speaking from experience. What exhaust are you running?