Sizing Outboard for Sailboat

[sailingisfun]

Re: Sizing Outboard for Sailboat

I need some advice on the size of motor for my sailboat. It's a 17ft. compac.
Weighs maybe 1000 lbs. It's really a small boat. I would only use the motor for getting out and away from the dock, and back again. Any suggestions?

 

[northernmerc]

Re: Sizing Outboard for Sailboat

If that were my boat, I would see if the Merc 2.5 that I already have would fit on it. That should be more than enough power, providing that the shaft length works.

 

[tino]

Re: Sizing Outboard for Sailboat

I can put up to a 40hp on my lafland 17 picnic sailboat

 

[bowman316]

Re: Sizing Outboard for Sailboat

i have a 7.5 2 stroke on my 26 ft full keeled boat, and it will get her up to 7.5 mph.

 

[sailor55330]

Re: Sizing Outboard for Sailboat

This is an old thread, but the information is usefull and worthy of revisiting.

First, and as External Combustion (we miss you, buddy!) correctly pointed out, hull speed is hull speed. It doesn't matter if you are "steaming" into a current, or riding with it - your engine can either achieve hull speed or it can not. Current affects speed over the bottom, not speed through the water. Those who fly airplanes will understand this concept quite well, because the headwind, tailwind scenario is the same.

Second, yes, you can stop motion of a displacement hull with thrust. The trick is to limit the speed of the motion, to something that the available thrust can overcome, in a distance that serves your needs. I used to back a Columbia 34 (13,000 lb displacement) into a marina slip regularly. The engine was an Atomic 4 and I had to maintain enough speed for steerageway in order to do this. That required use of the engine to stop motion, once I was lined up with the slip and part way into it.

What is more important to me about size for a sailboat auxiliary engine, is wear and tear on it. The OP went with a 9.8 hp engine in an 8,000 lb boat. I would have put the 15 in it, for this reason. Having 15 hp, even when you don't need it most of the time, allows for more thrust at the times that you do need it, plus you can run the motor at less than WOT most of the time.

In all fairness to a complete discussion, you also have to factor in seas, which can affect speed through the water, beyond the basic hull speed equation. Having a little more HP in that case can be very useful.

All of this said, my guess is that he chose the 9.8 for size/weight, at least to some degree. I've also had experience with a sailboat (Columbia 26) that had a well in the cockpit for the outboard. You can't tilt the motor up, so removing it and storing it in a lazerette, is the only way that you will get the boat to perform to its capabilties, while under sail. Frankly. doing that is a pain and the bigger/heavier the motor, the more of a pain it is.

One more thing on current v. hull speed. One of the skills necessary to properly handle a displacement hull, is to know what you can do with it and what you can't. When entering a leaving harbors, sometimes you have to pay attention to tides. Trying to buck a 6 knot tide in a boat with a 5 knot hull speed, obviously isn't going to work too well. If you are an experienced sailor, however, you plan for a tide in your favor, or a slack tide.

Know the boat, handle the boat!



°¿°

Good Post---as a longtime sailor, I agree with everything in here. I didn't see the displacement of the boat, but if it's 8k hull weight, displacement has to be around 12-14k. You'll need the 15 if things get ugly. For the non-sailors, you have to remember, a motor is for maneuvering in and out of harbors, setting anchors, and when the wind is unsafe or unavailable. It's not a primary means of propulsion or speed.

I am surprised that it doesn't have an inboard auxillary instead of the outboard. I would have bet money on an atomic-4 in that era.

Good Luck!

 

[GA_Boater]

I'm surprised you posted in such an old topic. If you would read the entire topic, you would have seen the the OP posted in September 2008 that he was pleased with his outboard choice, so your advice is only 6 years late. Please do not reply to topics older than 90 days.

 

[sailor55330]

For some reason, the post showed up on my screen as a new post. Maybe a glitch in the site? Perhaps my thoughts could be beneficial for someone just coming to the forum. Either way, my apologies. I'll go back to lurking and not sharing my knowledge, especially in a forum that doesn't get as much traffic.

Sorry for the confusion

 

[pvanv]

Good Post---as a longtime sailor, I agree with everything in here. I didn't see the displacement of the boat, but if it's 8k hull weight, displacement has to be around 12-14k. You'll need the 15 if things get ugly. For the non-sailors, you have to remember, a motor is for maneuvering in and out of harbors, setting anchors, and when the wind is unsafe or unavailable. It's not a primary means of propulsion or speed.

I am surprised that it doesn't have an inboard auxillary instead of the outboard. I would have bet money on an atomic-4 in that era.

Good Luck!

The HR28 was always a semi-custom build. Different interiors, either tiller or wheel, and option of either A4 or outboard located in the lazarette compartment. The original buyer (new) wanted to race a lot, so opted for a short shaft OB that he could tilt up into the lazarette (and then close hatch doors to the hull). Since we mostly daysail and cruise, the long shaft was a much better option, in the event of hobby-horsing in lumpy waves. Likewise, running a Remote model makes the setup perform as an inboard.

In the end, the 9.8 has been plenty adequate... but only because the Solas/Tohatsu HT prop does so well. That prop is a very good cheat. We are envied by neighboring sailors with similar setups who run 15 hp OB's and can not back as well.