Increasing WOT RPM

[146C]

Other than to reprop is there another way to increase WOT RPM.

I get a maximum of 20MPH at 4000RPM

The prop fitted is a Solas 13.5 X 15P

The motor is a 91’ DT85 Suzuki fitted to a Nova 5m Cuddy Cab.

The motor is a little difficult to start when cold but generally runs good thereafter.

Don’t know the total weight of outfit and don’t have any pics yet.

Not sure if it is a motor problem or just needs a smaller prop pitch(I think there is only one available @ 13.5 X 13P).

Any help would be greatly appreciated.

Dave.

[Blackened]

G'day

If 4000 is WOT, you need to come way down in prop size, FNQ will be best to advise on this

Dave

[BM]

Something is wrong here. A 17pitch prop should be reasonable on that hull, a 15 inch prop should be great. You are around 1500rpm shy of where you should be and as such you are damaging the engine due to overloading.

I would be checking compression, spark and fuel delivery before going any further.

Cheers

[146C]

I had the same thoughts BM, will check fuel and spark tomorrow. Had a compression test about a month ago and was told all 3 cylinders were the same 125 each.

Dave.

[FNQCairns]

Agree with BM there could be something mechanical wrong with it, but also certain other factors could gang up like to give you low numbers (although 4000 is very low) like leg way low, a lot of weight overall and forward also poor use of trim but best to sort out the mechanical options first as it has the potential to junk the engine fast depending on the cause.

Think Roughasguts has your engine on a like sized boat, not sure...

cheers fnq

[146C]

Spark is blue and fuel delivery to the carbies seem fine.

I also put in a new set of plugs today.

What else mechanically could I look at to help rule out a mechanical problem.

I did notice that a small amount of air seems trapped in the pimer bulb this is particularly noticable when squeezing it. Is this normal?
The primer bulb is brand new and just been installed a couple of weeks ago.

Cheers,

Dave

[FNQCairns]

There are a number of things that can point to an engine not producing full HP but first have you selected full throttle at the controls then viewed if there is any extra further movement available at the throttle linkage?

cheers fnq

[FNQCairns]

It may be worth double checking all fuel line connections by pulling then remaking them with an eye toward air leaks.
9/10 the simple things are the culprit, although it can something more complex.

Do you know your leg ratio?

cheers fnq

cheers fnq

[146C]

Hi FNQ,

Yes, throttle travel was one of the first things I checked.

I replaced the fuel hose from the seperator to the motor when I replaced the primer bulb, the hose was showing signs of cracking.

Have yet to thoroughly inspect the hose from the underfloor fuel tank to water seperator but at a glance seemed fine, cheap enough to replace it however.

Gear ratio is 2.08 (27/13)

Dave

[FNQCairns]

Hey Dave, how close are the numbers you gave? Very precise or just general? Was the speed GPS etc

Because I just ran them with your leg ratio and the outcome points to a prop/setup/trim/weight problem in any order/combination, not a mechanical problem.

cheers fnq

[gone_fishing]

sounds like a prop prob to wieght issue
depends if the prob has always been there or just devloped
of what youve been saying in tests see if you can get your hands on anther prop to test run
there is a way to increase wot rpm bigger hp motor
all the best

[146C]

FNQ the numbers came from tacho and speedo gauges fitted to boat.
I'm assuming they are accurate.

When using the motor trim I can get another 2 miles faster but no noticable difference in RPM.

Weight could be the problem, the boat is quite heavy. There is a fair bit of marine ply in its construction and it does not have much of a vee shape in the bottom of the hull, the bow also seems much more broader than a lot of other glass boats I have seen much like a tinnies bow.

gone_fishing I have not had the boat all that long.
It wasn't until I got the tacho working that I realised I was only getting 4000rpm.

Unfortunately no one I know has a prop I can try.

I will have to buy one hence the reason for the questions.

Suzuki lists only one prop available for my motor with a smaller pitch 13.5x13p.

Do you think dropping down to a 13 pitch would bring the rpm up near the 5500?

Does anyone else know of any other options I have regarding prop sizes.

Thanks for all the advice it is greatly appreciated.

Dave

[FNQCairns]

Dave speedos are usually very inaccurate, suspect yours is reading high at that speed, shame because it pointed well away from an engine problem.
Back to the start, the 13.5x13 will increase revs but it will be a slow boat, a right pain really, any prop with 15p and you will be happy to have on the water, the deal is to find a prop with less diameter (Like 13.25 to increase rpm) but stay with 15p as a minimum if at all possible, will put lots of money in your pocket over time with fuel prices the way they are.

Send roughasguts a pm about his engine I think it is a dt85 we could use his prop and numbers as a benchmark to estimate where you should be.

In view of the speedo as the speed reading it does sounds like a mechanical problem somewhere unless the speed is correct but that would be a bigger assumption than assuming it is wrong.

Things like engine height can make a couple of hundred rpm difference at this speed so can trim and so can extra weight or a lot of weight forward.

My engine is a 90yam 3cyl and has a 13.5x15 (5100rpm)cut down to 13.3x15p + cup taken out(5650rpm) on it, both engines have the same leg ratio and mine pushes a 6m tinny CC, so if it were my engine and using your prop getting 4000rpm I would be heading head straight toward the engine to rule that out first.

Unless of coarse there is something else that stands out like weight/legheight etc.

cheers fnq

[Splash]

There are a number of things that can point to an engine not producing full HP but first have you selected full throttle at the controls then viewed if there is any extra further movement available at the throttle linkage?

cheers fnq

FNQ -Best way to check for potential extra movement at throttle linkage without removing throttle cable to link?

D-A-V-E - do you have an additional fin on your motor (just above prop)?


SPlash

[FNQCairns]

FNQ -Best way to check for potential extra movement at throttle linkage without removing throttle cable to link?

D-A-V-E - do you have an additional fin on your motor (just above prop)?


SPlash

Splash probably no better way, can get pretty close just by visual inspection.

cheers fnq