Propellor size problem

[irawnsley]

The Boat:

Tradewind 35
10 tons
35ft loa
Perkins Perahma 35 (34hp)
hurth gearbox 1.79:1
propelor 17ins x 9in pitch

The Problem:

Engine max RPM 3100 (out of gear)
Engine max RPM 2000 (in gear) the lack of revs makes her difficult to turn in tight confines as there is so little prop wash.

Have used prop calc (http://www.surfbaud.co.uk/wave/) and this gives ideal prop size as 15 ins x 11ins pitch

No problem so far, just change the prop you say, however have spoken to a profesional prop person in the trade who says don't believe the downloaded prop program and that the current prop (17 x 9) is indeed the correct size.

Before you ask I have checked the fuel flow (by passing the system with jerry can and there is no problem with supply, the engine runs sweetly with no smoke.

/forums/images/graemlins/confused.gifAny ideas please!!!

 

[Aja]

Just a guess, but I would go with fitting the largest size prop - 17" seems ok.
However, The pitch seems a little on the light size.

My Moody 346 at about 6 1/2 - 7 tons all up has a 16" x 12.5" folder with a 33bhp (continuous rated) engine and a 2:1 gearbox.

In gear I get about 2000 revs, but plenty ooooomph when turning.

All subjective of course....

Donald

 

[brianrunyard]

According to Castlemarines PropCalc you need a 14"x10"

 

[sailorman]

15 x 11 & 17 x 9 are really the same thrust

take 1 inch from dia & add to the pitch = the same thrust

always have the largest Dia you can & reduce the pitch
my engine is rated @ 3000 rpm free unloaded spins at around 3500 rpm
i have the Maxprop set to give 2800 max rpm

 

[boatmike]

Difficult one. You can't just interchange diameter for pitch as one post suggests. The diameter is mainly a function of HP available. Pitch x RPM should give speed at no slippage. the slippage will be less with a bigger prop but over diameter at too high revs and the poor little engine won't turn it. So the statement "always have the biggest prop you can" is only correct up to a point. My guess is that you would be better off with a smaller dia and slightly more pitch but there is something about your statement that worries me. The fact that the engine will reach high revs with no load is meaningless. It should do that without full throttle. In fact I would not want to see full throttle with it out of gear at all! When it is under load I would expect black smoke if the revs are restricted to that extent so my thought is simply .... are you getting full throttle? With the lever fully forward is the pump throttle lever on its full travel at the engine end? I recon you may be operating on 3/4 throttle and assuming it's flat out when it isn't. It would also be useful to know the boat speed in calm conditions when you are at your maximum 2000 RPM... Theres something not quite right here...

 

[Topcat47]

a drop from 3500 to 2000 rpm under load suggests and engine problem to me rather than a prop problem, although it might be.

Hull speed for a Tradewind is a little over 7 knots and IMO a 35hp deisel ought to manage that with ease.