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Is there a "max hp" for a sailboat outboard?

rhbj03

Member I
When I was shopping for a boat I came across one seller who said he recommends "no more than 10 hp outboard engine" for his 25' sailboat.

I'm wondering, other than the physical size of an engine might not fit some wells, is there any limitation on how big an outboard I can put on a sailboat?
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
We owned two prior boats with outboards before moving up to the Olson.
AFAIK you have the basic issue of how much hp is "enough". Our first boat was a trailerable 20 footer with a new 6 hp Johnson which we called the "buzzer" because it made more buzzing noise than thrust (and was amost worthless in reverse).
:(

For most 25 footers, up to around 4 or 5K displ. 10 hp should be plenty. One thing we found was that the 4-stroke engines tended to have more torque at lower rpm (where a displacement hull form needs it/ uses it) than equal-rated 2-strokes.
As they say, "YMMV."

And then, with 4 strokes, a very few are designed for displ. hulls rather than small planning boats.
We had the prop pitch reduced on our first Honda 10, which helped a lot. Then we got a 10 hp "Hi Thrust" Yamaha and were amazed at the quantum leap in usable power. Same half gallon per hour consumption on both engines, BTW.

Mounting the engine is the other big limiting factor. You need a bracket with enough strength, bolted to a portion of the transom with... enough strength.
:rolleyes:

If in a well, the motor has to physically fit the boat, and the clamping area has to have... enough strength.
Some problems are endemic to outboards in general -- alternators that put out 5 or 10 amps on their best day (!). We found that either of our 10 hp outboards would not quite keep up with normal electrical demands while motoring... instruments, vhf radio, and AP. About every third night we needed to spend at a marina to plug into shorepower and recharge...

In short, for a 25 foot boat, 8 or 10 hp should be enough power, easily.

My .02,
Loren in PDX
 
Last edited:

rssailor

Moderator
No limit

I own an Ericson 25+ and have used a Yamaha 9.9 with the thruster prop and it is by far the best setup for a sailboat. Am currently using a Honda 15 that I bought used. It is a good motor and I changed the prop to a four blade with the lowest pitch offered by honda. Lower pitch is more power at slower speeds. Really the limiting factor is not the motor, but the boat in mounting the motor on a sufficently beffy bracket and making sure the transom can handle it. Use common sense of course when sizing motor to boat. Hull speed on 25 foot sailboat is generally around six knots. Hope this helps. Ryan
 

rhbj03

Member I
Yep I got it. Thank you both. I won't be moving my other 75 hp motor to the sailboat in hopes of making it go faster.

Just today another problem developed. The pull string on my sailboat motor seems to be wearing out. The outside skin completely separated, exposing the white fiber inside. I'm not sure how many more pulls it will withstand.

Have you folks changed the motor pull string before? Is this something I can do myself?

Robert
 

rssailor

Moderator
Pull string

Yeah of course the pull cord can be changed by anyone with a bit of patience and some mechanical know how. You might want the service manual for the motor as it might have a hint or two. Otherwise go to your dealer and order a new pull cord or make one yourself and then change it. Good luck and be careful with the spring as it is highly wound. Ryan
 

Sven

Seglare
Bigger != better

I wish I could give credit where due, but I don't remember the source:

'You don't want an extra large outboard engine on a sailboat because outboards tend to foul plugs if always run at low throttle.'

Or some words to that effect. Sure you can always run the engine at full throttle even if that will only get you a few 10ths of a knot more, but that's a very expensive way of cleaning plugs.


-Sven

PS On our crossing to Catalina this weekend we found out that not even a long shaft is enough to keep the prop under water when heading into steep breaking seas !
 

mdgann

'76 E23
Outboard size

There are several formulas that you can find online to help you size your auxilliary power needs. The one I have found useful is the minimum power to acheive hull speed. It is: HP= (Displacement)/((150^2)/(Hull speed^2)). For my E23 at 3200 lbs with a hull speed calculated at 5.7 knots this gives a minumum horse power of 4.7 to acheive hull speed. This is an ideal calculation. In practice my 4.5 hp Mercury pushes the boat at about 5.5 knots at full throttle. I think that I would like to have about a 6 hp to take care of bad weather motoring. I like the size of the 4.5, but could use alittle more oomph when the wind and waves are really up and I have some difficulty with the sails. It hasn't happened yet, but it could. Good luck. M.Gann
 
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