Propellor Pitch

[Biggles]

My boat is fitted with a Tohatsu M40C 2 stroke outboard.

At present I attain the highest speed at 3/4 throttle. Any more application of the throttle and all that happens is that the engine gets nosier but the boat does't go any faster.

My GPS says I reach a top speed of 30mph and the boat planes very quickly and easily. The maximum boat design speed is 42mph.

How do I make use of that power that doesn't seem to be getting used.

I have been informed that I need a higher pitched propellor but I'm not too sure what the figures mean in the motors book. IE which propellor I should be changing to.

I think my engine has a Mark 11 Tohatsu propellor which in metric is 268mm dia X 287mm pitch.

The other Tohatsu options that are available are as follows:

Light Boat to Heavy Boat.

Mark 14 260mm dia X 352mm pitch - Light Boat

Mark 13 262mm dia X 322mm pitch

Mark 12 268mm dia X 296mm pitch

Mark 11 268mm dia X 287mm pitch - My Engine

Mark 10 275mm dia X 252mm pitch

Mark 9 276mm dia X 226mm pitch

Mark 8.5 285mm dia X 220 pitch

Mark 7 290mm dia X 180 pitch - Heavy Boat

Any ideas on which one I should go for.

My boat is an Avon Seasport Deluxe 3.6m with quite a lot of accesories(Weight).

Also is there anywhere to bye these props off the net at a reasonable price as I can see me buying a couple of them until I get it right.

Many Thanks in advance for any advice.

[Louise]

I'm sure the prop experts here will help you regarding what sizes to consider, etc, but as to buying a couple of props to try, if you can wait until June, Julie from 'Steel Developments' (prop specialists) has said they will be having a stand at RIBex where they will be offering a try-before-you-buy service.

HTH.

[Graham H]

Hi I cant advise what prop but what I did was go to my local marine shop (Bill Higham in Worsley) & asked him for advice after a chat I bought a secound hand prop . He said if dosen`t have the desired affect come back & try another, as it was the one he recomended did the job.( Saves buying props) hope this helps

[Diesel]

Ok,

What you have to do is prop the boat with your usual load in it, say two people.
You have to get the engine to rev to its maximum rpm at full throttle which i think is about 5300rpm.

For example if you have say a 13" pitch prop and it revs to 5100 you wany to go down a pitch to 12" to give you and extra 200rpm.

The rough guide is about 200-300rpm for each pitch of prop.
A smaller prop will give you more rpm and a bigger pitch will give you less rpm.
You also have to consider pick up speed etc but thats another story!.