Need a New Propeller, Please Help

[andreshs1]

Hi

How can I know what propeller would suit my boat best??

I have a Shipman 28 with a 20 BHP Yanmar, but I am sure the propeller could be improve, any tips??

Do you have a similar engine?
What propeller do you use??


Thanks for taking the time to reply

Regards
Andres

[daddle]

One starts with two pieces of valuable info. First, what is on there now? Second, what is the performance now?

The performance check is done with the boat in proper clean condition. One should be able to get nearly full power rpm tied to the dock (or stern anchor for proper sailors) and full power rpm at cruise speed.

Without that info one must rely on an identical boat or a numerical computation.

[Target9000]

Also please note your transmission gear ratio.

[TOM]

There are a number of good prop calculation websites ,they will ask you to enter things like boat lenght,gearbox ratio ,displacement etc,etc they give a fairly accurate answear .I used one when i repowered .

www.byamee.com

[Astrid]

If you write to prop manufacturers and give them information found on the following questionnaire. The manufacturer will then know precisely what kind of prop you need tailored to your boat. While the question sheet is for a feathering type prop, the information requested will do for any prop, fixed or otherwise.

Size Questionnaire

Also check out this page:

http://mysite.verizon.net/resqp86c/p...lers/id20.html

[andreshs1]

Hi guys

Thanks for the info and the links. I'll get on the water this weekend Nd get the measurements

I am very new at sailing, only had the boat for a few months, so I dont know much about boats.

What propeller do you have installed or you recommend??
folding, fixed pitch, self pitching, high vortex, etc

Regards
Andres

[Astrid]

I use a VariProp.

[Cheechako]

The Max Prop is probably the most well proven variable pitch prop out there. Doesnt mean others arent good. But it will cost you probably $2000-3000 US. Otherwise a 3 blade Michigan is good. at probably $300 (?) You need to do the RPM test though.

[SkiprJohn]

Here's something that's been around the block a few times that might help. Read all the way through and check out the graphs.
http://www.cruisersforum.com/forums/...size-5972.html
kind regards,

[Boracay]

Changing propellers is an expensive complicated process. You really need to check the performance of your existing propeller first.

My suggestions: -
1) If you don't already have a GPS get one. Plotters are best but a plain GPS that reads in knots or, even better, tenths of a knot will work fine.
2) Next time you go out check your GPS speed and engine RPM. Make notes in your log. Check your engine as you go and don't use more than 80% power unless you have to.
3) After you get a feel for how the boat is look for a quiet calm stretch of water and do a couple of runs up and down it at various engine speeds. Average the speeds in each direction. Make note of the results.
4) Now calculate your boats theoretical maximum hull speed. This is often taken as 1.4 times the square root of the waterline length in feet.

Now, did your boat get anywhere near its hull speed? Half a knot is probably not worth worrying about. More than that and you've got something to chase over the next few years...

[Astrid]

As a thought, how clean is your hull and prop--that would affect how well your boat does on a trial run.