Max Horsepower Rating 20 ft sailboat

Apr 26, 2015
660
S2 26 Mid On Trailer
One point appears to have been missed on HP rating and that is your engine mount. Most have weight and HP ratings. Some of the older ones were designed for 2 stroke engines and not 4 stroke. 4 stoke has more thrust initially and will bend some of the old mounts at the same HP rating. Also hanging too much weight on the stern of a boat is a good way to slow it down.

If you use Dave Gerr's formula for hull speed prediction, you may go a little faster with the a slightly larger size outboard than the average 4HP on a 20' boat.
 
Sep 25, 2019
38
CATALINA C27 Lk Norman NC
Years ago in a galaxy far far away, my father ran a Townie in the Merrimac with a 3hp seagull, and it would do well even against tide and current combined, so I think 4-6 hp would be perfect. I've not messed with the torqueedo , but have heard good reports. my 2 cents
 
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Sep 20, 2014
1,320
Rob Legg RL24 Chain O'Lakes
I have a suggestion, since you have considered 2 outboards, I would suggest getting a 24 volt trolling motor 85 lbs thrust. I used one on my RL 24 and it worked great up to about a 25mph headwind. At that point, I needed the 4HP gas engine. The great thing about using a trolling motor is that it always starts. Its dead quiet. its lightweight. You have much better control when docking. If you are waiting in line at the launch ramp, it is very easy to just bump it on and off to keep your position. You don't have to sit and idle or shut your engine off and hope that it starts if you start to drift into another boat. A trolling motor will be cheaper than a Torquedo, and will do just fine for what you need it for. Its also nice to ghost along quietly or even tow the kids through the water on a hot day when there is no wind. Most of the time I was sailing on a 20K acre lake, so that was several miles long. The trolling motor worked just fine, except when I got into a 40+ mph wind storm. Then I lost forward motion.
 
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Jan 19, 2010
12,362
Hobie 16 & Rhodes 22 Skeeter Charleston
Forgive me if I got this wrong but it seems there is a fundamental missunderstanding about the difference of a planning boat and a displacement boat. Sailboats are displacement. A displacement boat has a max speed that requires extraordinary HP to overcome.... so in general terms there is a limit. For a 20' boat your water line length is about 19' so your hull speed can be estimated as

HS = 1.34 x √LWL

... just shy of 6 kts. or just shy of 7 mph. You will hit that at about 1/2 throttle on a 6 HP O.B. Any more throttle and your gas is being burned to push a larger bow wave. You won't go much faster but you will burn more gas. You want a little reserve in the throttle for when you are driving against a head wind but two outboards is overkill. You got sails so your sails are your back up... or if you are a sailor, your O.B. is your back up.:beer:
 
Aug 10, 2020
511
Catalina C25 3559 Rocky Mount
i ran a 3.3 merc on my little 21. more than adequate. i run a 9.8 merc on my 22. it is overkill, but reverse is handy
 
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PaulK

.
Dec 1, 2009
1,223
Sabre 402 Southport, CT
It sounds like the OB salesman may be concerned about LeeandRick's point about the mounting and transom. You're not going to be waterskiing behind this boat: a 40-horse engine might simply rip the transom off. Putting two outboards might have the same effect. It might be better to stow the spare forward, if you can. It might help balance out the other one on the transom, though the weight of two motors is bound to slow you down a lot in any case. Others have given good answers to your horsepower questions: there's no point in over-powering it. No one seems to have mentioned the need for a long shaft to try to ensure that the prop stays in the water in a seaway, however. Honda makes a 9.9 long shaft (25") that seems to be quite popular. While it may be more hp than necessary, it may be helpful on long slog in a seaway, as has also been mentioned.
Taking the mast down to go faster would not be a good idea. Studies show that the inertia of the mast helps to dampen the rolling of the boat. Much like a trapeze artist who carries a long balancing rod, the mast helps steady boats that, when they lose the mast, roll more and sometimes roll over.
 
Apr 26, 2015
660
S2 26 Mid On Trailer
For a 20' boat your water line length is about 19' so your hull speed can be estimated as

HS = 1.34 x √LWL

... just shy of 6 kts. or just shy of 7 mph. You will hit that at about 1/2 throttle on a 6 HP O.B. Any more throttle and your gas is being burned to push a larger bow wave. You won't go much faster but you will burn more gas. You want a little reserve in the throttle for when you are driving against a head wind but two outboards is overkill. You got sails so your sails are your back up... or if you are a sailor, your O.B. is your back up.:beer:
Or you could use 8.26 ÷ D/L ratio↑.311 (Gerr formula) in place of 1.34 and come out with a possible higher speed that could be attained with a little more power. My old S2 goes faster than it's supposed too using the standard formula.
 
Oct 24, 2010
2,405
Hunter 30 Everett, WA
In my old powerboat days, I had a 115 and a 4 hp kicker for fishing. The only time I needed a second motor the flywheel key sheared. I was down to 2 bad motors. On our old Macgregor 26D we had a Honda 9.9 and I added a Honda 2 for the dinghy. To carry it I added a motor mount on the transom. The 2HP would push us over 5 mph easily. The 9.9 would make 7 plus mph on half throttle. One tume I got caught in a strong tide and fired up both motors. It was hard to tell if I got more speed adding the last 2 hp or not and that was being right next to a navigational buoy so progress was easy to see. We passed it at about a minute per boat length.
 
Sep 8, 2020
48
Merit 22 Honker Bay
You mention going 7 miles off shore to an island. You may find that no matter which engine you have that if the sea is running your propeller will be out of the water a lot.

I personally don't think it's a good idea to depend on an outboard for anything more than getting a sailboat in and out of a harbor.
 
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Jan 11, 2014
11,323
Sabre 362 113 Fair Haven, NY
You mention going 7 miles off shore to an island. You may find that no matter which engine you have that if the sea is running your propeller will be out of the water a lot.

I personally don't think it's a good idea to depend on an outboard for anything more than getting a sailboat in and out of a harbor.
And when becalmed. Drifting around in the hot summer sun is no fun. If there is wind, sail it will always be more fun and usually faster.
 
May 24, 2004
7,129
CC 30 South Florida
Do not bother looking for a rating plate. For the years that your boat was built most available outboards were 2 stroke engines which carried a different power to weight ratio than the 4 stroke available engines of today. The point is that the recommended HP ratings in the mid 1970s could today exceed the weight carrying capacity of the motor mount and affect the boat stability. Think about how the boat is going to be used; in a protected lake you may use a smaller engine but if for use at sea or river you will want a stronger one. Based on experience with similar sized boats and different engines I would say get an engine that weighs no more than 60 lbs and the 6HP engines fit that bill. These outboards come in two models, a Long and Short Shaft; make sure you get a long shaft as they will keep the prop in the water a lot better than the short shaft. As far as engine performance there is nothing more important than a properly sized prop for the combination of your boat/engine. Outboard engines are usually shipped from the manufacturer with a middle of the road propeller that may or may not fit the characteristics of your boat an engine combination. They usually default to a higher prop that may not allow the engine to deliver maximum RPM and offer a slower acceleration. Unless you purchase from a seller that may have the ability to allow you to test different size props it is very hard to determine which might be the optimum prop for your setup. The correct prop will allow the engine to rev up to but not exceed the maximum rated RPM. The same engine on different boats will behave differently because of hull shape and weight considerations. My recommendation is a 6HP, long shaft. The long winded explanation is to give you food for thought about motor mounts, weight, stability, horse power, props, hull shape and displacement and most importantly your specific needs.
 
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Oct 22, 2014
20,995
CAL 35 Cruiser #21 moored EVERETT WA
Your boat has a 17.5 water line length. That give it a theoretical hull speed of 5.6 knots per hr. The boat is rated at 2000 lbs displacement.

My experience with a 15 ft boat used a 2.5hp Mariner 2cycle gas engine to power the boat at hull speed of about 4.5 knots. It’s displacement weight was 750 lbs.

My opinion is you will be well served either a 4-6 hp 4cycle outboard. I would consider leaning to the 4hp due to weight.

Your primary power are the sails. With good sails your motor is no better than 6 very large rocks that you picked up walking the beach. They are dead weight for a sail boat. So the lighter the engine the better.

Sailing is the journey not the destination.

Good luck with your quest.
 
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Jun 8, 2004
10,024
-na -NA Anywhere USA
Suggest as a dealer 4-5 hp for lake sailing and 5 to 6hp for coastal. 5 go either way.
RK stood for Robert and Kay as that boat was built in the Shenandoah Valley, VA
They went out of business. I use to own an RK 20. Later I became a sailboat dealer
 
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JRT

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Feb 14, 2017
2,037
Catalina 310 211 Lake Guntersville, AL
Please don't buy 2 motors and start doing goofy things like taking a mast down. Get more comfortable sailing your boat, stay close and build your confidence. Buy a new motor and I would highly recommended Tohatsu from this online dealership. I've bought 2 from them and they were just great easy motors and company to deal with.

If you have enough money for 2 motors buy some new sails instead of a 2nd motor, all used old boats with old sails improve with good sails.

 
May 17, 2004
5,032
Beneteau Oceanis 37 Havre de Grace
I figured that taking the mast and boom down, in the case that I had to motor to shore from the islands quickly, would help compensate stability wise for me raising the keel,
Counterintuitive, but taking the mast down is generally considered to reduce stability, not increase it. Having that weight extended at a distance acts as a damper for rolling. As long as there’s enough righting moment to compensate for the weight.
 
May 24, 2004
7,129
CC 30 South Florida
Thank you everyone for the information. This might be crazy but I'm thinking of having 2 outboards. Most of the time I wouldn't use both but having 2 makes me feel more secure. I want to go about 7 miles offshore to some islands eventually, so reliability and speed are important to me. My boat has a swing keel, so when lifted, and with mast and boom down, in an emergency, how much hp would be safe and how fast could I go? How much is hull speed affected by having the keel up?

Do not, repeat, DO NOT attempt to sail with anything less than the swing keel at least half way down. This will seriously impact the stability of the boat and cause it to roll and flounder. A sailboat is a perfect hybrid as if the engine quits you may continue to move under sail. Never considered or needed a 2nd engine. Hull speed is a theoretical number based on an old formula using a constant value (1.35) which was derived for the hull shape of old time sailing ships. It still renders a fair approximation in boats with classical shaped hulls. The significant premise in the answer to your question is that in a displacement hull is that to exceed the theoretical hull speed you need significant increases in HP to attain small increases in speed. I know that in a small boats we all seek speed to get to our destinations but in a 20' boat it is really not attainable with any significant increase. You theoretical hull speed based on your hull wetted surface of 17.5' yields 5.5 knots. I find this a little on the high side for a 20' boat so anything between 5 and 5.2 knots would be more realistic. speed can be measured "over the water" and "over the bottom" but readings must be taken in two runs in opposite directions and averaged out. This is to neutralize the effects of wind and currents.
 
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Sep 20, 2014
1,320
Rob Legg RL24 Chain O'Lakes
I will say one more thing about the 2 motor/trolling motor option. When I did this, the 4HP was stashed under the cockpit. This helped to keep the weight low, which made for good ballast. It also kept the heavy motor off the transom. Sailboats have a narrow transom. As such, putting a heavy motor on the rear of the boat makes the rear ride low. This affects your speed, both under sail and under power. The cabin makes for a shorter cockpit and forces everyone to sit near the rear of the boat, which is also doing you no favors. Using a trolling motor, you can put your batteries anywhere - the best place being either side of the swing keel. You will want to run 8 ga wire to the batteries. That should be enough for 50 amps, which is all the trolling motor pulls. Also make sure to put a 100 amp breaker at your batteries, just in case anything shorts out. My boat was over 2K lbs fully loaded. With a 5 position speed setting, most of the time I used position 3. That was fast enough to cruise along and feel like you were making good headway. My family also felt more confident, knowing they could lower the trolling motor and starting it meant twisting a knob. If something happened to me, they knew they could handle the boat. The gas engine not so much.
 
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Feb 5, 2009
255
Gloucester 20 Kanawha River, Winfield, WV
A 20 ft sailboat will move quite nicely with a 4HP outboard.
Mine does.
I own two outboards, a Yamaha 4MLHP and an Evinrude E4RLCUD. Both are two strokes from the closing decades of the previous century. The Yamaha has a better "hole shot" which I think is mostly a function of its propeller, while the Evinrude with it's tiny pair of pistons is hands down the smoother and quieter ride. I seldom need to go beyond a quarter throttle with either one. Any more than that just adds a lot of noise and splashing without much noticeable increase in speed. I've found either motor to be remarkably dependable, which is a pretty big concern for me because I'm not a very good sailor and I operate around a lot of barge traffic.
 
Sep 24, 2018
2,549
O'Day 25 Chicago
Thank you everyone for the information. This might be crazy but I'm thinking of having 2 outboards. Most of the time I wouldn't use both but having 2 makes me feel more secure. I want to go about 7 miles offshore to some islands eventually, so reliability and speed are important to me. My boat has a swing keel, so when lifted, and with mast and boom down, in an emergency, how much hp would be safe and how fast could I go? How much is hull speed affected by having the keel up?
I had the same thought when I first started sailing my old 19' sailboat. Most outboards a quite reliable. Mine had issues starting but it always started eventually. I had a trolling motor on the rail as a backup. Only used it once when I forgot to check how much gas I had before going out. Most of the time motors are only used to get in and out of marinas, slips, moorings and storms so I wouldn't worry about a backup auxiliary too much. With a bit of practice you should be able to sail into a lot of slips
 
May 24, 2004
7,129
CC 30 South Florida
The reason I recommended a 6HP engine over a 4HP is that for most manufacturers 4,5 and 6 HP are the same engine (size and weight) but with a different carburetor and a higher degree of tuning. Obviously the 6HP will cost more but the extra power may come in handy in foul weather. I remember cruising in the Long Island Sound on a 22' boat with a 9.8 HP 2 stroke engine and only moving forward to our destination at 1 knot until the current slackened. make sure you purchase the long shaft. We had previously been running with the current at 8 knots until it changed on us. Planning around tidal changes can and will affect your speeds when traveling.
 
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