Is my prop the wrong size?

[Mozza]

Hi all,

My new boat with the 40hp Merc 2str on the back just ain't performing the way I expected it to and the suggestion has been made that the prop may not suit the setup.

The boat is 4.3m long, she's got a 4mm plate bottom, 3mm plate sides and a 3mm plate deck so ahe's heavier than the average tinny. Its a tiller steer boat and struggles to get push 2 burly blokes and an esky full of fish. To my ears, it sounds like the revs could be higher. Would prop size affect the ability of the motor to get up to decent revs?

The prop is a SS Vengeance 10.5" dia. prop with 13 pitch (part#48-855858A).

Any suggestions?

Thanks
Mozza:-/

[dreemon]

Hi , yes it does sound like a heavy boat and if you and your mate are big the prop size would need to go down in pitch to maybee 11 p? it would be a real help if you could get an rpm reading, tinnies tend to like a bigger diameter blade on the prop, yours being 10.5 might have something to do with it and getting to much slip ? engine height may be low and adding drag, while on the plane have a look to see if you can see the cav plate sitting on top of the water , a foil like permatrim or SE 300 will help some boats heaps and fail on others, , I would try to find out the rpms first, good luck

[FNQCairns]

Assuming a wot of 5500 and a 2.0 gearbox if you cannot reach 47km/h then yes that prop is too big, how too big will depend on the difference between the speed above and your actual GPS speed. Just a 5km/h difference here a large difference on a boat and will stuff a 2st engine.

cheers fnq

[Mozza]

Thanks guys,

My top speed is ~30kt, rpm I dunno - it just sounds a little low, like it's not hitting the 'powerband'. I'll check the cav. plate but, I think it was professionally installed so I'd be surprised if the leg is too low.

Dreemon, a lower pitch blade sounds logical and fnq, yeah, my impression is that the motor is under some strain. I've got a guy coming on Wed to have a look so it's good to have this info so I don't sound like a total dunce.

Ta heaps.
Mozza

[FNQCairns]

Hi Mozza to pull 30kn the engine would need to be spinning at 6K or better so right up there in the rev range, was that GPS? Still I am assuming a 2.0 gearbox which I think should be right but am not definite.

If the gear ratio and your speed is correct you cannot go down in size actually up is your only direction if 6K is to far above the manufacturers recommendation but up in pitch will make load carrying even harder or it should.

cheers fnq

[Richo1]

I agree with FNQ on that one. 30knots is pretty good for that size motor and boat, might have to go a bigger motor to do any better. Just put as new prop on my boat (went up in pitch) and lost 1 knot producing less RPM but fuel economy is better.

[Mozza]

Thanks again guys,

The merc. website has a prop-size calculator which has just returned a recommended prop/pitch of ~11"/11º for heavier loads. The gear ratio is 1.83.

That's where the main issue lies - with just the 2 of us, not a problem. Add another 100kg of ice/fish/gear and suddenly she struggles to get up on the plane. I just had a great trip to Missionary Bay (Hinchinbrook - you wouldn't know the spot fnq;) ) but had to endure a 2 hour trip there AND back because she just refused to get up. Even after we used/jettisoned the fuel, water, excess ice etc.

A compression test will tell me if the motor's r/s otherwise I'll have to invest in another prop or, hydrofoil to help get her onto the plane.

Cheers
Mozza

[Mozza]

Hi again,

I'll just wipe the mud from my face and admit that 30kt is highly unlikely and I was probably reading the GPS in kph not kts!

That might make his thread and the original question a little more believable. Thanks dreemon.

Mozza

[Noelm]

no were talking! 30KPH is probaly more like it, with the result that your prop is way too big, can you borrow one from someone for testing? is the rig new to you?

[Mozza]

Yes Noelm,

I can confirm that the speed was 21kts (not 30) as I've just checked with the deckie who has a better memory than I.

When you say too big, do you mean diameter or pitch or both?

Thanks
Mozza

[Noelm]

probably a combination, I would see what FNQ comes up with, he has a handle on all things props, if you could borrow some props for testing would be great, if you could find out your max RPM, that would be fantastic. A lot of people get sucked in with the thinking that a big prop is better, because it lowers your RPM, that is only partially true, if your Motor cannot acheive max manufacturers RPM, then you are slowly but surely killing it, and wasteing fuel, not saving it!

[FNQCairns]

The new gearbox ratio at your maximum speed with the 13p your rpm is 4300! No wonder your having trouble it's a barge not a boat!, a radical prop change is needed.

The divide you need to cross is to big to guess with any sincerity no matter who you talk to, please don't put a foil on now, that's a trap lots fall into, and esp if you head on relatively long trips nearshore, no foil will mean longer range and money in your pocket by comparison.

Anyway will the merc prop seller in your area allow you to test and swap?? If you can find a 12p that works you will do 27kn flat out (assuming 5500, and the safe assumption of 12% slip), if an 11p then 25kn. Like Noelm says borrowing is beaut!

As a best guess for what it's worth at this point, what can you find to try that is a 10 in diameter and 12 in pitch??? ANYTHING 11 diameter will disadvantage you no matter what RPM you run it at, that's a given but sometimes a necessity, still it's time to suck it and see what will work best through testing.

Oh yeah, probably best not to run it again until a smaller prop is fitted, the risk of doing costly and permanent damage with such a low WOT is very high. Also the leg is the right length for the transom??



cheers fnq

[Mozza]

Thanks heaps fnq!

Jeez, I hope the damage isn't already done. I'm oof to the FM grounds tonight so hopefully I can borrow one for that.

Cheers
Mozza

PS Are you still going out today?

[Noelm]

you will be safe enough to run it, but not at full throttle for a long time, to fully explain what is happening to your Engine, your Motor has a manual timing advance, that is, when you turn the throttle, it mechanicaly moves the timing to advance it, now in your case the Motor is doing no where near what it should be at full throttle, so, with the timing fully advanced, the spark plug is firing way before the Piston is at the top , and the resulting explosion (firing stroke) is trying to push it back the other way, but another cylinder is not allowing this to happen, now this is not a good thing, very hot explosions in a confined space (and getting smaller) does bad things to soft metal (like pistons and alloy heads)

[Mozza]

Woohoo!

NQ O/B wreckers will lend me one for the night - its a 12pitch not sure of the diameter.

But anything's better than what I've got.

Thanks again.

Mozza

[FNQCairns]

48 38098A1- approx 10.25x11

48 38094A1 - approx 10.25x12

Good luck, see if they have either of these genuine merc props (or the equivalent cross reference in another brand...Turning point??) to borrow/test, the 11 will probably give RPM closer to 5500 than the 12 although a search for a 12 of the right diameter to pull 5500 is a worthy quest.

cheers fnq

[oldboot]

It is rely worth buying the genuine paper manual specific to your motor.... problay arround $50.
I am relay impressed by the merc manuals,

It should have wot recomendations and prop tables with part numbers.

That would be realy helpfull when trawling for used props.

Remember to look at props of smaller and larger models similar to your motor.

My motor a 30 HP shares prop format with similar from 20 to 65Hp......If you know that and have part numbers... it gives you a big advantage in the second hand market.

Some bloke have this stuff in their head......for the rest of us prop table are a real help.

That manual will tell you all sorts of things about your motor to.

cheers

[Richo1]

Hmmm, Missionary Bay which creek # 1,2,3,4,5? Found #2 to be the best in the past :)

[FNQCairns]

Mozza did you catch anything and what did they give you?

cheers fnq

[yamp]

mozza
I bought one of those small digital tacho from a mob called christie engineering on the net.It was really useful when I was stuffing around with props on my boat.its was a good gadget shows revs when motor is running and hours when stopped I think it cost around $70 I left it on just zip ties onto the tiller arm cheer Yamp

[Mozza]

Thanks for the continuing help guys.

The shops down here are all saying that I'll have to go up in diameter if I want to go down in pitch??

No luck as yet on sourcing an 11" borrow but I'm working on a few contacts. I can get an after market SS prop @ 11.5" 13P for ~$550 which sounds reasonable but just need to test first.

Yamp, good idea. Probably get one anyway.

Richo, it was a 1st for me so the learning curve was very steep. Gonna post in the F/B but the bro' came back with a nice barra.

Mozza

[yamp]

mozza
give solas props a ring I used them and was really happy with the advice given, pretty good prices cheers Yamp

[aussiebasser]

My 50 EFI 4 st would push a 435 Hornet at 72kmh with a 13" Vengance and no weight aboard in salt water. Put two people in it in fresh and it wouldn't get on the plane. As these are not high performance, or high torque motors, it's almost necessary to have a couple of props. I'd try an 11 or 12" for when ya fat mate's aboard.:-X

[dreemon]

[

The shops down here are all saying that I'll have to go up in diameter if I want to go down in pitch??


Yes thats is right , by about 1/8 - 1/4 in for every inch in pitch, so if a 13 has11 1/2 d , an 11p might be 12 or almost 12 in d, but alot has to do with prop manufacturers all being a lil different and if it's s/steel or alloy, if you get a prop that is a little to small you can have it cupped to bring down the rpms, good luck