articfriends

iTrader: (1)

Got to test my boat under ideal speed conditions this weekend,looking for advice because boat is definately going SLOWER. Here is the story-272 baja single w/950 hp/ft lbs tq,peaked at 5700 and only drops a few hp to 5900,tq peak at 4700. With 1.50 drive with nose cone and stock prop centerline to hull dimension of 7 1/2 inches boat would run (89.5-89.7 on gps) at 5900 with merc labbed 4 blade 30 pitch prop, max trimmed, with 4 passengers and 3/4 tk of gas. Ran in the 91.5 to mid 92 range with light load and 2 passengers with a best of 92.8 mph turning a merc 32 pitch 4 blade (labbed by a unknown shop) at 5700 rpm's.I tried running the 32 labbed with more weight,boat lugged hard while accelerating and boat ran 89-90mph at 5400-5500 rpm's. I installed a stellings box this year in the 1 inch up position,boat porpoised,cavitated,wouldn't carry the bow,blew prop loose around 3000 rpm's,would only run mid 85-86 w/30 pitch at 5900 w/full load,ran 89.5 once with light load carrying the bow well but couldn't really get it to carry the bow enough to repeat it. Tried the 32 labbed,ran 87.5-88 on most runs struggling to carry the bow,ran around 90 once when it did get aired out.
Added a 1 inch drive spacer a few weeks ago,seems to carry the bow well but gets real loose and squirelly in any kind of cross chop (coimpared to before without the box). Never got any good speed runs until this weekend,had light load and 32 pitch on,boat ran 84.7 mph at 5200 rpm's on several runs,put 30 pitch labbed,boat would run 87.5-87.9 touching rev limiter at 6000 occasionally so I don't think motor is hurt or lost any power.Here is my question-aside from pulling motor and the box and going back to original stock set-up that obviously worked better than what I have now,should I try removing the drive spacer and running a 5 blade prop? Another bit of info,boat will tq hard to left if weight isn't perfect when running 32 pitch labbed until bow airs out and even worse when I tried a 32 merc 4 blade non-labbed so I don't want to waste my time with a 5 blade if its going to tq twist even worse to where its almost unsafe. Should I try a 28 or 30 5 blade,should it be labbed,thanks,Smitty

BenPerfected

It is difficult to measure the results and do a valid comparison with different fuel and passenger loads. Even varying air densities can impact the results.
As the B1 30P seems to be the best all around prop, maybe you can make each run with exactly a 1/2 tank of gas with two people in the boat and see if you can get back to the 90 MPH range. After you are good with this base line, that would be the time to compare it to the 5-blade prop in the same conditions.
Question...is the prop shaft in relation to the bottom of the boat exactly where it was before the extension box was added? If not, that may be a goog place to start.

articfriends

iTrader: (1)

Yes,the prop centerline is back to 7 1/2 inches (the original depth) from adding the 1"drive spacer. The only real difference is it is 12 inches back from the stellings box which puts it in different water and the extra 100 lbs the box,driveshaft and steering ears added. The boat ran 89.5 to 89.7 with a pretty good size load probably 50 times last season and at least 10 times this season with the 30 pitch at 5900 touching the rev limiter at 6000 once in a while before adding the box,in different temps anywhere from 70 to 90 degrees with 70-75 degree water so it's never been real fussy when I have the 30 pitch on.This weekend I was only running 40 gallons of fuel,water was 66-68 degrees so inercooler was working real well,there was nothing extra in the boat and air was cold and dense. The rpm's were there,just not the speed,so I'm pretty convinced the ext box,with or without the spacer has done nothing in my case. I'm debating pulling spacer back out and trying a 28 or 30 pitch 5 blade but I don't want to waste any more time or money if its going to tq twist real bad or not gain top end. I think there will be stellings box for sale slightly used in the near future,Smitty

Pure Energy

I have a 28p Maximus that I used once, will give you a good price on it.

I have a 30' Superboat that will tq hard to port when going over swells spinning it RH. I found this out when I tried the Maximus. Used it for about 20 min test time. I normally spin it left and have no issues. I was told the RH would do this by the previous owner, but had to try for myself.

Don't know how your boat will react, but I have a 28p 5 blade RH Maximus if your interested at a good discount.

BenPerfected

I can't see how the extension box is likely the issue. More thoughts...any porpoising issues now vs. no ext. box? Have you done a leak down test to confirm ring and valve seal?
If you can't find the solution as rigged, I would go back to the dyno this winter before I would spend more $ potentially chasing set-up and prop ghost.
What if you re-rigged and then found out that you still couldn't find the speed? What if you are down 100 HP (10%)?
$500-700 in dyno time may be the low cost solution.

articfriends

iTrader: (1)

Porpoising-the boat porpoised after installing the box very bad,it used to porpose slightly and slowly between 65-75 if you tried to cruise there before the box. The drive spacer seemed to get rid of 80% of it but its squirerlly still and porpoises in rough water between 60-80 if no longer accelerating now unless I dig the tabs in harder than before the box and even then it still does it.I also thought the motor could possibly be getting tired or is hurt from the fact it could only pull the 32 pitch to 5200 rpm's but when I put the 30 pitch back on it would pull 5900 which is definately strange as there has never been that big of a rpm difference between the 2 props. The reason I keep thinking the set-up is still slowing me down though is I made 10-12 passes and races saturday and sunday for 45 seconds to a couple of minutes with different trim positions and tabs pulled up and tach was at 5900 and I could occasionally feel it touch the rev limiter at 6000 for a brief millisecond and gps readings were only 87 to 87.9. with most barely over 87. The boat (before the box and spacer) would show 89.5-89.7 with another 500 to 700 lbs in it at those same rpm's,same conditions & trim and tab settings w/30 pitch.If I pull motor to remove the ext box I'm definately going to freshen it as good measure. I'm still missing a few mph compared to my rpm's,boat is definately still handling worse then before, still porposing some in rough water and is squirrely in rough water/cross chop (it was squirrely before because its only a true 25 ft long,just riding on the pad and is over powered but nothing close to this). Now back to my question,I'm still considering removing the 1" spacer and trying a 5 blade before the end of the season but I have heard 5 blades on single vees typically torque twist like a beetch so its probably a waste of time,what pure energy experienced is what I'm thinking I will see too. I'm hoping the prop experts might chime in but its a bag of snakes to make a solid reccomendation to me at this point so I don't blame them for staying out of it . I may call a few places tommorrow,I would like someone with exerience in set-up to say-No,a 5 blade will only go slower and twist your boat all over the place or yes,the slip and porpoising will be gone with our prop at the plus 1" raised position and do what your looking for,Smitty

bbladesprops

Good stuff. I need to go though this again and run some numbers/thoughts and I'll get back to you.

BenPerfected

If you are porpoising more now, it is likely because you have moved the CG to the rear with the drive now hanging on the 12" box. Where are your inner strakes in relation to the CG? You might ask a few more people than me, but an easy test is to make some 6-12" oak wood strake extensions and screw and gel coat them in place and go back and make another run.
The trick is to make them just long enough to get the boat stable. What is likely happening is as you trim the boat, you are now running out of stakes to run on. The boat is now squirrelly (running on flat bottom) and falls back on the bow and then starts to lift again...porpoising as it runs off and then on the strakes. This is exactly what my boat did when I added the boxes. The Mercury crew at Lake X helped me figure it out.
Yes, you can give up the box and move the CG back forward, but your will give up the opportunity to really gain some speed with the elevated X and lower HP case benefits. I can ask a few others, but i think if you could potentially go even higher on the X if you had your balance right. You can keep changing the strake length until you get it just right. Now you get a FG man to make the extensions in glass...not a big deal.
This test takes $100 worth of test material and some DYI labor. It is all about the balance...
How bad do you want the speed?

articfriends

iTrader: (1)

Did some more testing last night (thanks to a fellow oso'er). Baselined boat up and down river,40 gallons of gas,cooler,500lbs of passengers(my normal light load ). Boat ran 87 upstream w/30 labbed merc at 5900,87.2 downstream with 30 labbed merc at 5900 touching rev limiter 3 or 4 times at 6000. Bolted on 28 pitch merc maximus 5 blade non-labbed,ran 86.5 at 5700 (my hp peak),lagged from 5000-5700 noticeably,clunked very hard going in and out of gear (stellings box with driveshaft and no coupler). Tq'ed to port even with bow up slightly,chinewalked a little even at wide open which it never does. Had phenomenal cruise though-3-4 mph faster than 30 labbed at 3000,3500 and 4000 rpms (prop slip calculated at 11% less than normal at all those speeds). Going to try it tommorrow without the 1" drive spacer but I don't think its going to miraculously be better on top end,we'll see,Smitty

articfriends

iTrader: (1)

Tested boat again today with drive spacer removed which puts prop shaft up 1" above stock dimension with 5 blade 28 maximus. Boat drive fairly good even in rough water,no torque steer but I had a pretty good load in it. Ran 86.5 mph gps again so no extra top end. Was no metal in drive oil when I drained it to remove spacer.leaned on it 6 times or so at wfo,was heading in and opened it up slowly till 5000 then leaned on it to wfo,hit 5300 and sheared something off in the drive . I think with the tq I'm making a 5 blade is just too much strain. I have inspected drive several times this summer,everything has been looking real good with the exception of the lower gears which I am experimenting with,its been 3 years since I sheared off a shaft or gear,in the back of my mind I was wondering if a 5 blade with its low slip factor was going to be the straw that broke the camels back. I'm tearing drive down in next few days,feels like a vert shaft but not sure yet,Smitty