What best guess max speed from a new prop

On a small 18 ft with inboard, I have just installed a new Yanmar 30YM20 (21 MHP) 16 kW at max 3600 RPM , gear ratio reduction is 2.21 giving about 1628 rpm at the prop. LWL is about 14.6 - 15ft feet, draft about 2 ft, not sure of loaded displacement weight is but Im estimating this at 1000kg. Existing prop is 14'' but do not have pitch yet. I have a few questions:

From an earlier post it would appear (please see photos, that this is semi displacement boat, most respondants said so and some said it was a full planning boat boat without at least 80-100 hp would not plan (accepted) ,but my question still is to get the best from my new engine so looking for some experienced advise on this )

Q1. Will a semi displacement boat (assuming it is) with this engine spec ever achieve above max hull speed of a displacement boat (5-6 knots) with same spec driven by a new 21Hp engine with this gear ratio ? If so could I get to 8-10 knots?

Q2. What would the optimium prop size and pitch be for this boat ?

Q3. What options might there be to get the boat above displacement speed by way of changing prop. (prop was driven by old volvo 20-25hp engine which max rev was about 1900 rpm, seems this achieved about 5-6 knots but now need to change form anti clockwise prop to clockwise prop.) Hence I want to max my speed given new investment. Getting the prop config is very key. I have seen some sites which calculate these metrics but Im looking for an old sea dog with experience to comment on their opinion.

many thanks in advance

Paul (Gilaroo)

 

Thats a planning hull with shaft. It would plane with 100 probably less, say 50.
Youve done it all wrong, you decide what speed you want , then choose the prop to do it , then chose the engine to drive that prop.

I would start with something in the ball park like a --9x9 for your 20HP.

Check out your local prop man he will no doubt tell you what you need and probably have a few for you to try.

 

That is very much too small! Even an outborad optimized for ~20 kn has a ~10" diameter.

Joakim

 

Joakim

Prop calc application from Castle Marine suggests
best prop is :

1. If full displacment hull 13.34 X 8.68
2. If Semi displacement 13.76 X 11.56
3. If Planning hull 13.76 X 10.97

I'm not going to have enough power to plan with 21 HP but do you think
I should go for semi displacment configuration in hope I can get above
max displacement speed of 5.35 knots. The calc app indicates therorically
I could get to about 9 knts?

Paul

 

I still say 9x9. Read this.

http://www.woodenboat.com/forum/showthread.php?t=90743

 

I didn't see much related to this from the link given.

Wageningen B-series propellers (3 blades, Ae/Ao=55%, 10% wake factor):

A 9x9 propeller would give a thrust about 600 N and require less than 6 HP at 6 kn speed and 1628 rpm shaft speed. At 10 kn it would have a thrust of about 300 N and require about 3 HP (much better efficiency).

With a 14x9 propeller the figures would be about 2000 N and 20 HP at 6 kn and 1150 N and 13 HP at 10 kn. So this would be good for 6 kn, but too small for 10 kn.

A 14x11 propeller would be 1900 N and 23 HP at 10 kn and 2800 N and 31 HP at 6 kn, thus close to at 10 kn but way too big for 6 kn. At 6 kn it would rotate a bit over 1400 rpm and have thrust of about 2000 N.

A 14x10 propeller would be 1500 N and 17 HP at 10 kn and 2400 N and 25 HP at 6 kn, thus a bit too small for 10 kn and a bit too big for 6 kn. At 6 kn it still would take more than 1500 rpm and have a thrust of more than 2000 N, thus it would not be worse than 14x9, if you just don't overload the motor.

I would not go to anything smaller than 14" in diameter, if you just have enough room (at least 5 cm from tip to hull).

Here is some interesting reading: http://www.fao.org/docrep/x0487e/x0487e04.htm

Joakim

 

Joakim

from what you said I would read it that 14X10 might be best option then 14 X 11 ? More of concern is I only currently have 2.5 cm (1 '') to spare at the top of current prop to hull which is 12.77 '' and 4.5 cm (1.75 ") to spare from tip on bottom. If I put a 14" prop in I would only have about clearance of 1 cm on the top and 2.5 cm on the bottom, what are the consequences of that.

The current prop with current clearence of 2.5 cm top and 4.5 cm has lasted
30 years so far ?

 

Frosty

found it hard to gain anything from that link , appeared to be relevant for a gas engine not inboard diesel set up. I think 9 diameter is def too small for set up. Thanks for replying.

 

Too small clearance will result to noise, vibration and reduced efficiency. Here are some absolute minimums from the book by David Gerr:
http://books.google.fi/books?id=8w09O4hooWUC&printsec=frontcover#PPP1,M1

Noise and vibration can be reduced by having more blades and/or skewed blades, but 14" seems clearly too much for your installation.

Joakim

 

Thanks Joakim for all your help.

I'm going to trial a 13" and increase the pitch to 12 and see how I get on. That is a very comprehensive book and extremely useful reference.

Paul