Prop size

[Gordonmc]

Be gentle... I have done a search and cvan't get an answer and have tried various prop calculator sites... I don't have enough information for them to work properly. as I don't know the slip of the respective props.
If anyone has experience of replacing a prop I could do with some wisdom before taking this project any further.
The boat is a 9 metre long keel yot with a 35 horse T90 powering a three-blade 12 x 8 right hand prop. Gearbox is a velvetdrive 1:1. I have never been impressed with her speed, around 4kt at 2500 revs and the engine never seems to be working very hard. Upping the revs leads to black smoke and not much more in the way of velocity.
I have been offered a 15 x 14 right hand prop which will go in the rudder cut-out.
Discuss.

 

[ex-Gladys]

If a 12x8 is giving you an "overload" (i.e. black smoke and not reaching peak revs, the 15x14 is going to do even worse. It's a coarser pitch and bigger diameter. I would suggest that because you have a 1:1 gearbox, you need to go to an even lower pitch.

 

[ebbtide]

CONTACT CLEMENTS MARINE - WHIZZ KIDS - OR PM ME.

 

[boatmike]

My first question is are you sure your gearbox ratio is 1to1? If so I would think at 2500RPM your pitch is far too high. IMO with little else to go on your prop would seem to be suited to a GR of 2to1 or higher. Forget diameter for the moment and stay with pitch. If your prop RPM is 2500 then the pitch should be about 1/2 what you have or less assuming a hull speed of about 6 knots. If you really do have a non reduction gearbox the choice of prop for optimum revs is more critical than with reduction and to get it right first time will be difficult. You should be able to reach max RPM without labouring or black smoke though. You say it doesn't labour but smokes. What is the max RPM of the engine? What is the maximum you can achieve? In any case the 15X14 is far too big. Confirm what type, displacement, etc boat it is, and the gearbox ratio and we might be able to guess a rough optimum but something sounds very wrong here as 35HP for a 9 metre boat is quite generous.

 

[chriscallender]

I used this calculator very successfully when reengining, and it estimates prop slip from other parameters

http://www.castlemarine.co.uk/pitch.htm


But I also really wondered about the 1:1 gearbox ratio. If its really 1:1 the shaft is spinning at quite some speed and using some guess numbers in the propcalc software I came up with the optimal prop size being something like 10x5.

In that case, maybe the 12x8 really is overloading the engine, which would explain all the black smoke. But I'm thinking then that the only way your going to get decent performance is to change the gearbox rather than (as well as) the prop.

Well, I'm no expert, only someone who changed an engine and am very happy with the result of the one prop/engine matching that I have ever had to do.


Hope the calculator helps.
Chris

 

[oldharry]

If its a 1:1 gearbox the shaft speed is way too fast, the prop is cavitating and losing most of its power. For a notional prop size calculation, use a slip figure of 45% for a dispalcement hull.

Shaft speed needs to come down to a maximum of around 1800rpm for a displacement hull speed of around 7 knots.

Simply increasing the prop size will make matters worse - even heavier load on the engine which is overloaded already by the 12x8. IIRC the Throneycroft engine should run at around 3500rpm (anybody know for certain?) to develop 35hp, in which case restricted to 2500rpm it probably only gives around 12hp which is not enough for your hull, and is why you get such low speeds.

Re-propping will not solve the problem; you will not get full power unless the engine can run up to near full revs in gear, with a 2:1 box and shaft speed around 1750rpm together with a suitable prop around 14x10 (assuming 3000+RPM as the top engine speed)

 

[boatmike]

Well Old Harry I think you have said roughly the same as I did but there are such things as 1 to 1 gearboxes and the problem in this case can be solved by re-propping but as I said, halve the pitch. It is better perhaps to reduce the gearbox ratio and have a courser pitch prop but wrong to say you cant solve the problem by changing pitch on the prop at all.