O/B prop to power yacht

[Seal_surfer]

I have a Tohatsu 5HP 2 stroke as auxilliary power for my 22' cruiser. There doesn't seem to be much bite or urge in spite of it being in a well, set at a reasonable depth and checked for vertical. I realise this is a modest power for the application.
Question is - would I gain much extra thrust by changing the standard propellor for the optional lower pitch version intended for heaviier displacement craft. Is it worth the £60 or so being asked, or should I put towards a more powerful O/B?

Any experience of trying different O/B props on same boat would be useful.

Many thanks,

Phil

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[chippie]

If your outboard easily reaches maximum revs you could possibly get a bit more pich put on the standard prop for not a lot of money. If you have a brochure for your motor there may be a power and torque graph printed which will show where the maximums of these are developed. I am not familiar with your boat so cant tell whether the motor is well matched to the hull or not. I have been using a measly 6hp outboard to get my 34' trimaran on and off the mooring but she is relatively easily driven . 9.9 hp would be better.
I recently downloaded a propcalc program from castle marine which could give you some figures to work from.

Good luck

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[wooslehunter]

5hp should be OK. If the engine doesn't reach max revs and sounds laboured, then a finer prop could help. Generally, a finer pitch, and possible larger diameter will give more thrust at lower boat speed.

Here's some techno-bumph. The engine's power curve is peaked. A small course pitch prop has to be moving faster through the water to rotate at the same speed as a larger but finer prop although both take the same power to drive them. Therefore the prop should be selected so the engine revs at peak power at the boat speed required. This gives best use of the engine power although not necessarily best efficiency. For best efficiency use the sails.

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[Boathook]

Normally on an outboard the prop is designed to push a fairly fast craft. On a sailing boat or other displacement craft the hull speed is quite a bit lower. It sounds like you need a finer pitch. Most manufactures do a selection of props to cater for all boat speeds and after 20 years with o/b's on cats it is worth the expense.

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[Athene V30]

I am not technically minded but I have a 4hp Mariner 'sailmate' (ob has a larger propellor than standard) and it drives my 22' fin keeled sloop at 4 knots in smooth water.

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[snowleopard]

a prop designed for a fast boat will not work efficiently on a slow boat as the angle of attack of the blades will be too steep. to get the most thrust you need a finer pitch. problem then is that the engine isn't working so hard so to compensate you need a larger diameter. there's not much scope for larger diameter on an outboard because of the cavitation plate getting in the way but you should be able to get an extra inch at least.

when fitting my props i was advised that 17x11 and 16x12 would both put the correct load on the engines but the 12" pitch would give more top speed on flat water and the 11" more thrust in choppy conditions.

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[Seal_surfer]

Thanks for your replies.

Whist 5 knots is curently no problem in flat water and moderate wind, it is a bit of wind and chop that really bring out shortcomings. his als when you most need that extra urge.

i'll have to see if the "saildrive" pro option is a bigger diameter as well as finer pitch.

Thankyou again.



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[max_power]

I largely suffer from mechanical impotence on subjects like this, but I do know that Pat Manley, PBO’s engine guru , thinks that outboard props/pitch are overlooked on sailboats and should match the boat on an individual bases, i.e hull type, weight etc. He says that in 99% of cases we worry about HP too much when we should think more about the prop.

I think we also often expect too much speed from engines ( even taking hull speed into account) when compared to sail power. I remember reading that boats are designed on the basis that 50 sq feet of sail represents 1 hp in a force four wind. Although racing boats are much lighter generally than cruisers, I think we overlook this fact sometimes when it is a simple fact that pro rata, they have much more sail up for a given hull length. Sorry to wander off the subject a bit.


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