Engine size

pcv

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I have just perchased a 20ft day sailer with swing keel and would like advise on size of outboard engine to use. The boat is at Western S Mare on the Bristol channel,
many thanks
ps Dont want the boat to look like a speed boat
 

oldharry

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I used an 8HP Honda 4 stroke on a 20foot trailer sailer. Normally running at around half throttle which was quiet and economic, and with the extra power to spare when needed to buck a foul tide (4+ kts on springs) when around 7/8ths throttle seemed to give max waterline speed. The little extra bit only made more waves, but was useful in strong headwinds
 

david_e

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Depends on many things, some pointers; if you don't have to move it around alot the the advice for four stroke engine is good because of the greater amount of/and smellier fumes that a two stroke emits, also the fuel ecomomy is generally better. Down side can be the weight of the unit if you have to lift it etc. If you don't need alot of power then definately go for a smaller size engine you could probably get away with 4-5hp, in which case read the tests in some of the back issues of the mags to help you narrow it down. Buy locally where you can get warranty back-up. Difficult to be precise without knowing your boat type and anticipated usage - what are they?
 

JamesS

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Just to add to below - when I was looking for an engine for my previous boat it was suggested that the size of engine should be about 4hp per ton of boat.

Hope this helps.
 

claymore

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Might be worth considering whether you want it to have a battery charging facility. The old Mercury Saildrive was 7.5 h.p. (I think) and that was about the lowest hp engine with this facility. As James S says boatweight is an issue - I had a Micro 18 a few years ago which was pushed along really well by a Yamaha Malta (3.5).
 

jollyjacktar

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Bigger is better

I had a similar size yacht and replaced a 5hp with a 10hp... gave me an improved safety factor of additional reserve power for trying conditions, a battery charging ability [ 5 amps max] or at least an ability to power up electrical things with the motor running. I could run it on about half throttle for hull speed where it was more economical and quieter than the 5 hp screaming its head off. The extra cost was reasonable. Still things may have changed since then.
 

PeterGibbs

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It is hard on a quiet day to imagine the difference between the power needs of a boat such as you have, and the demands when faced with strong winds and adverse currents. I would advise a power selection on the basis of the latter, and it will require a multiple of HP's over the needs on quiet days!

Even 4hp can move a significant boat in calm waters; I can shove my 8 tonne 38 ft Bavaria quite nicely with this sized O/B, but 10 times that is needed to achieve a safe landing in rough conditions. Windage is a big factor of course, and a light vessel may not have the weight itself to make progress against even 2-3 m waves. Best be cautious and enjoy the quit hum of the (underused) engine on the normal shunt out from the moorings to the sailing grounds!

You should be looking at no less than 20 HP. Possibly 25.

PWG
 

gary_yank

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I use a 10hp on my 23 foot trailer sailer. The area we sail in, Puget Sound,US, has a moderate tidal flow and we can get some heavy winds. The 10hp is a 4 cycle and is rather heavy when compaired to the lighter 2 cycle 6hp we had, the increased weight does mean that I had to shift gear down below for proper trim. The large size is nice in that we go hull speed at half throttle and fuel economy is good. The larger motor also has a charging system that will top off the batteries.
Look at the boats in you area with a size and underwater shape as yours, If the boat has an owners group check with them. The weight of an overly heavy motor will cause your boat to sail off of her designed water line.
 

oldharry

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20 - 25hp!!!! Firstly, the designer will have determined a maximum practical engine size for the boat, and will have engineered the transom accordingly. It is unlikely to exceed around 15hp, and could be dangerously overstressed if a 25 is hung on it and run hard, partiucularly in a seaway. Check with the builders if they are still around.

This of course assumes the hull is not designed to plane under power. If it is, then it will take a much larger engine, and be much more strongly constructed. But the sheer weight of even a 25hp+ engine could have very unpleasant effects on handling under sail.

A 20 foot daysailer in calm conditions should go quite adequately with around 4-5hp. Double that will keep the boat going against pretty well anything you meet. If you really insist on going out when conditions are at their worst, then possibly 15hp (with the extra weight and fuel consumption to take in to account as a minus) might just be justifiable.

Anything more will just be a waste of time, money and fuel on a 20 footer, simply making larger and larger wash as the hull digs deeper into the water (and your pocket!) . A displacement hull can only go up to a certain speed through the water, dependent on the waterline length, and doubling the power will give very little extra speed, but at 3 - 4 times the fuel cost! And in a 20 footer you will likely get very very wet anyway as you try!
 
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