Sizing Outboard for Sailboat

[pvanv]

I have an 8,000-pound sailboat that uses a small outboard for aux power, and I need to replace the motor. This is a long shaft, remote control, electric start setup, in a lazarette well.

Originally, this was an anemic 9.5hp 2-stroke. Everyone has replaced them with bigger motors. We now have an old Evinrude 15hp (similar to a 9.9), which is adequate. Definitely not too big for the boat.

I would like to buy a 4-stroke Tohatsu/Nissan/Mercury. The 4-stroke 15 is a lot heavier, which is an issue, getting the motor in and out of the well. Also, the 4-strokes have a much larger cowl -- just look huge compared to the old 2- strokes.

Some dealers have told me that the 4-strokes have much better low end power than the old 2-strokes, and so the 9.8 has such good low end thrust that it would be fine. Other dealers say I need one hp per 500 pounds of sailboat, so 15 is the way to go. But I'm not convinced.

Will a 9.8 actually push my 8,000 pound boat OK?

Since price is about the same for the 9.8 and 15, what do you think? The new 15 is 20 cubic inches, while the 9.8 is 12. The old 15 was 15 cubic inches.

 

[tashasdaddy]

Re: Sizing Outboard for Sailboat

smae price go bigger.

 

[oceansoul63]

Re: Sizing Outboard for Sailboat

If you can get your boat to hull speed with the smaller one, then it would be fine.

After a sailboat reaches hull speed, any extra horsepower isn't really going to do you much good and will only serve to waste fuel.

That said, the 15 probably wouldn't be overkill with the boat you've got, so then it might be nice to have that extra bit of oomph.

 

[TOHATSU GURU]

Re: Sizing Outboard for Sailboat

No, the 9.8 and even a 15 is too small for your load. While hull speed probably will be achieved with an 8 or 9.8 that is not what you need for safe maneuvering power. Your weight requires at least an 18 at a minimum. Otherwise you run the risk of not being able to stop or get through a channel that has a 5 knot current.

 

[oceansoul63]

Re: Sizing Outboard for Sailboat

The 18 minimum does make sense, actually. I guess I wasn't really realizing just how heavy this boat is. What kind of boat is it?

However, I must say, respectfully, on the subject of "stopping," the physics are much different for a sailboat than for a power boat.

You're not really able to "stop" a sailboat with your engine, like you do with a power boat by throwing it into reverse. Especially not an 8000 lb boat.

Because once it gets momentum, it doesn't just stop. I've seen sailors with big boats coming up to the dock too fast, then trying to slow down with reverse. And it just doesn't work. When that happens, somebody usually has to jump onto the dock at the last minute and stop the boat with a line on a cleat or pole.

 

[External Combustion]

Re: Sizing Outboard for Sailboat

I'm lost with the logic of not getting through a 5 knot current. Hull speed is hull speed (maximum of 1.34 times the square root of the waterline length) regardless of whether or no the water is moving itself. It takes one HP per ton (2240 lbs) to reach 1.0 X sqr root wll and two HP per ton to reach the 1.34.

Most people seem to be happier with more horsepower and if you are fighting a strong headwind with your masts and rigging providing many pounds of resistance, then the larger engine would make sense.


Like Oceansoul63 said you will not be doing any battleship stops with the small propeller that an auxillary has, so that is not part of the equation.

If you motor a lot instead of sail then the larger one will be better. I know of a few sailors that have never tried to dock without power and a few that it would be very dificult for them to because of how their slips are arrainged at the marinas.

If time on the water is most important the smaller engine and smaller fuel bill will be adequate. If keeping a schedule is more important then the larger will come in handy at times to make maximum hull speed.

 

[oceansoul63]

Re: Sizing Outboard for Sailboat

I don't even have a motor for my Oday right now, so I'm learning how to dock it under sail. In light air that's blowing favorably, it's a piece of cake. But it can get dicey in big wind. Especially when I'm by myself.

Anyway, not meaning to go off topic. What EC said makes good sense to me.

 

[pvanv]

Re: Sizing Outboard for Sailboat

This boat is a 1968 Hinterhoeller HR28 fin keel sloop. Hull speed for this displacement (not planing) vessel, which has less than a 24-foot waterline length: Yes, the formula is indeed 1.34 x sqr-rt of waterline; is about 6 knots. The present, tired, 1975 15hp E/J does not usually achieve that speed. Just too much boat. But the present setup is adequate for how we use the boat on Lake Ontario, which can have ocean-like wave conditions. Backing is every bit as important as forward, and slow speed control is key, so a shallow pitch prop (6 inches or less) is used, and a 20" or longer shaft is mandatory to keep the prop in the water if the boat porpoises through waves. The steering is done by the boat's rudder, and the OB is locked in dead-ahead. No room in the lazarette well to turn it.

18hp is not an option. The 2008 Tohatsu lineup goes from 8/9.8 to 15/20, with each group based on a particular block. The 20 is way too big/heavy/expensive, and the 8 is way too small, so available options are 9.8 or 15. The 114-pound 15 has an oil filter, and generates 12 amps charging, while the 84-pound 9.8 has no filter (just a screen), and charges at 6 amps. But the 9.8 is 30 pounds (almost 30%) lighter. Price difference is about 10%, so the weight and performance are much bigger factors than cost.

If I go with the 15, it will likely stay on the boat year round, as it will be a hassle getting it in and out of the well. Plugs can be changed through the lazarette hatch. The lower unit lube and the motor oil can be changed annually, as well as the occasional impeller, when the boat is hauled and on the cradle for the winter each year -- with the motor hanging down below the hull. The 9.8 is attractive, as it is light, but the consesus is that it may not have enough thrust, even though some dealers claim that 4-strokes have a lot more low speed thrust than their 2-stroke counterparts.

Sailboat aux engines are sometimes more critical than their powerboat cousins. They are usually only needed to get in and out of the slip, or for getting home in complete calms. But when they are called on to power through a gale, they can be lifesavers. There's no small kicker to use when the main engine has a problem, because the main is actually the one and only small kicker. So I'm thinking that the extra reserve power of the 15 might be a good idea.

Thoughts?

 

[tommays]

Re: Sizing Outboard for Sailboat

Well

I am just getting my second J24 going which has to be about the most easy to move 24 ft boat built


The Current boat has a 1 cyl 4hp 2 stroke and while it moves the boat OK my last J24 had a 2 cyl 6Hp jonyrude which moved the boat MUCH MUCH better anytime there was current or other adverse conditions


So if your anyplace the water moves OR it gets windy beyond your comfort level the big motor provides a LOT of saftey factor


I got buy with a short shaft motor a LONG TIME until one day i allmost did not and bought that long shaft real FAST

 

[oceansoul63]

Re: Sizing Outboard for Sailboat

After reading all this, I gotta say I'd go for the 15, then.

 

[External Combustion]

Re: Sizing Outboard for Sailboat

12 amp charging and greater thrust will probably tip the scales (pun intended) for the larger engine.

You might consider a wharf side davit for pulling the engine for service.

I am glad my Baymaster 18 only has a 2 horse on it. It came with the boat and it is very easy to wag around the place for maintenance and storage.

Curiosity has the better of me. What is the fuel tankage for the engine? Integral tank, separate tank or both?

Fair winds and following seas to you.

 

[oceansoul63]

Re: Sizing Outboard for Sailboat

I just got an older 4 HP Evinrude longshaft for my sailboat (Oday 20) for next to nothing. Good compression, turns over, just no spark. Now, let's hope I can get it figured out.

 

[pvanv]

Re: Sizing Outboard for Sailboat

The boat has two Tempo 6-gal tanks. 12 gal at 2g/hr = 6 hrs of total running with the old 2-stk 15.

But, after reviewing all information from Tohatsu, it would appear that the 15 is not an option. Too big fore-and-aft to fit through the lazarette hatch.

So the only possibility is the 9.8 with high thrust prop.

Thanks to all who have replied.

 

[country_bumpkin]

Re: Sizing Outboard for Sailboat

No, the 9.8 and even a 15 is too small for your load. While hull speed probably will be achieved with an 8 or 9.8 that is not what you need for safe maneuvering power. Your weight requires at least an 18 at a minimum. Otherwise you run the risk of not being able to stop or get through a channel that has a 5 knot current.

Agreed if you have to deal with those variables. I sail on a land locked lake, so I don't have to deal with the current.

 

[pvanv]

Re: Sizing Outboard for Sailboat

The results are in. Went with the Tohatsu 9.8 Long Electric. Installed the new High-Thrust 4-blade prop. Did lose a little top speed, but can still make almost 5 knots. The tach and gps show about a knot per 1,000 rpm. Very quiet. Fuel economy is much better than the old 2-stroke: 1GPH at WOT. The AMAZING part is the low speed control. Backs into the slip like parking a car.

 

[tommays]

Re: Sizing Outboard for Sailboat

In this neck of the woods you wait or go backwards as it takes a really long time to buck 6 knots of current in a 7 knot boat

 

[jay_merrill]

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!



???

 

[tommays]

Re: Sizing Outboard for Sailboat

I moved and 8000# CAL 29 from the Throgs Neck Bridge out to Glen Cove through tanker traffic and all that stuff WITH 4HP at 4.5 knots with a pretty dirty bottom

I think the 9.8 will be FINE as despite the extra power of a sailing inboard the prop is a massive comprise between thrust and low drag when sailing

I also race on a C&C 35 that has a 13.5 HP diesel which pushes it to 7 knots and has gotten the job done in winds of up to 50 knots safely

Now the modern Chlorox Bottle floating sail/houseboats boats do require a BIG motor

 

[solarfry]

Re: Sizing Outboard for Sailboat

This boat is a 1968 Hinterhoeller HR28 fin keel sloop. Hull speed for this displacement (not planing) vessel, which has less than a 24-foot waterline length: Yes, the formula is indeed 1.34 x sqr-rt of waterline; is about 6 knots. The present, tired, 1975 15hp E/J does not usually achieve that speed. Just too much boat. But the present setup is adequate for how we use the boat on Lake Ontario, which can have ocean-like wave conditions. Backing is every bit as important as forward, and slow speed control is key, so a shallow pitch prop (6 inches or less) is used, and a 20" or longer shaft is mandatory to keep the prop in the water if the boat porpoises through waves. The steering is done by the boat's rudder, and the OB is locked in dead-ahead. No room in the lazarette well to turn it.

18hp is not an option. The 2008 Tohatsu lineup goes from 8/9.8 to 15/20, with each group based on a particular block. The 20 is way too big/heavy/expensive, and the 8 is way too small, so available options are 9.8 or 15. The 114-pound 15 has an oil filter, and generates 12 amps charging, while the 84-pound 9.8 has no filter (just a screen), and charges at 6 amps. But the 9.8 is 30 pounds (almost 30%) lighter. Price difference is about 10%, so the weight and performance are much bigger factors than cost.

If I go with the 15, it will likely stay on the boat year round, as it will be a hassle getting it in and out of the well. Plugs can be changed through the lazarette hatch. The lower unit lube and the motor oil can be changed annually, as well as the occasional impeller, when the boat is hauled and on the cradle for the winter each year -- with the motor hanging down below the hull. The 9.8 is attractive, as it is light, but the consesus is that it may not have enough thrust, even though some dealers claim that 4-strokes have a lot more low speed thrust than their 2-stroke counterparts.

Sailboat aux engines are sometimes more critical than their powerboat cousins. They are usually only needed to get in and out of the slip, or for getting home in complete calms. But when they are called on to power through a gale, they can be lifesavers. There's no small kicker to use when the main engine has a problem, because the main is actually the one and only small kicker. So I'm thinking that the extra reserve power of the 15 might be a good idea.

Thoughts?

The 9.8, 15, 18 and 20 hp Nissan/Tohatsu are the same block , weight the same and probably share all parts except the carb jets and exhaust manifold.

I had the 9 and upgraded to the 18. When I bought it they did not have the 20. They are the same weight, size. Different props and prices.

Get the 20 electric. Charge your batteries along the way

 

[TOHATSU GURU]

Re: Sizing Outboard for Sailboat

"The 9.8, 15, 18 and 20 hp Nissan/Tohatsu are the same block , weight the same and probably share all parts except the carb jets and exhaust manifold."

Not quite true. The 9.8 is a much smaller engine frame. The old 15 and 18 A & B models have completely different blocks than the new 15 and 20 C models.