Underpowered or wrong prop?

[JeremyF]

Please excuse raising this subject again, but I searched and couldn't find a posting on this subject a few months back....

Took a Westerly Oceanquest out for a test sail on Saturday (Sailed beautifully, and behaved just as I hoped in gusting winds) However I think its either a bit underpowered or the prop (two bladed fixed ? dimensions) is wrong.

The Oceanquest is 34.5 LOA, and 27 LWL. 6.4 tons. 28hp Volvo. At 2100 rpm it made 4.5 kts in a flat sea, and 5.1 kts at 2500.

This seems a little slow, and at 2500 it felt as if the prop wasn't biting fully.

I believe there is a program that helps calculate prop dimensions. Any idea of the URL?

Jeremy Flynn

 

[vyv_cox]

http://www.propworks.com/ for a downloadable prop program. I am told that it is not extremely sophisticated but it works well and gives perfectly acceptable results.

My Sadler 34 is very similar in size and displacement, with a Yanmar 27 HP. At 2200 on flat water I am doing more than 6 knots with a Brunton Autoprop two blade, from memory 17 x 13. This is a lot better than the previous fixed 2 blade, also undersized, I think a 16 x 12

 

[Chris_Stannard]

I used to have a Sigma 362, similar size and displacement. She did 6.2 knots at 2500 rpm on a Volvo 2003 which sounds very similar, with a two bladed folding prop. If you have the revs and no power it does sound like a prop problem. How about being cheeky and asking Volvo what they would recommend for the engine. You can translate a 2003 into the modern 2030 equivalent. Alternatively you could ask the sales man at someone like CJR in Southampton to recommend a new propeller to you. You could then see what they think should be fitted and compare with what is on the shaft now/

Chris Stannard

 

[Guest]

I have a 28hp 2003.

if it is a saildrive at 1:2.2 reduction ratio, then the two-bladed prop would be about 16x11 or 16x13.
If it is a lower-geared straight shaft, then maybe 17x12 (my dad had a 2003 on a Westerly once with this).

For what you describe at 2500 rpm, you would think the boat would do 6.5 knots. Is the hull clean? Something growing on the prop, or heavy hull growth would result in such performance (recent experience!).

regards

 

[JeremyF]

Its a saildrive, and freshly antifouled on a clean hull

Jeremy Flynn

 

[Guest]

That is the same as on mine (HR-312, about 5.5 tonnes at the moment - fairly long keel).

I had a 16x11 Volvo folding prop which did 7.2 knots at 3200 rpm (flat out), and cruises at 6.2 knots at 2200 rpm. It was buggered so I put a 16x13 fixed two-blade on it which was certainly no better, and possibly worse. I may go a 16x11 three-bladed folder, now that I have tried that little experiment!

The boat you are looking at is slightly heavier, but probably longer waterline. You may struggle slightly to reach max potential with a 2003, but it should perform fine at lower speeds (less than 7 knots) - so something is wrong. You need to look at the prop.

The saildrive has a higher gear ratio (mine is 1:2.2) than a straight shaft normally (say 1:2.7), so you have to be careful about superficial comparisons. It makes a big difference.

On the Volvo web site under the parts section is an extensive chart matching props to engines very roughly - but useful as they cover saildrives. I don't think I agree with it all the time, but it seems close enough not to matter that much.