13-footer Re-power with 20-HP

[davidc]

I have a 1968 Boston Whaler SPORT 13 to re-power. It currently has an old 40-HP engine. To save money on a new outboard and the fuel it will use, I was thinking of purchasing a 20-HP engine. [What is the] top speed [of a SPORT 13] with 20-HP? Thanks.

[jimh]

According to the REFERENCE article on the 13-foot hull, the minimum horsepower is 9-HP. This is assumed to be enough horsepower to get on plane with a light load. With 20-HP you should be able to get on plane. You will have to select the propeller pitch to be appropriate for the load. In small and light planing hulls, the speed is very dependent on the crew weight, since the crew weight is usually a very high percentage of the total boat weight. If the crew is two youngster, the boat will go well. If the crew is two guys over 250-lbs, the boat won't go so well.

[jimh]

If you buy a modern outboard engine of the same horsepower as your current engine, its fuel usage will be about half compared to your older outboard. If you buy a modern outboard of half the horsepower of your older engine, its fuel usage will be about one-quarter of the old engine.

In general, the cost of fuel to propel a 13-foot boat is not an impediment to its use. If you can afford the other costs of boat ownership--boat registration, trailer registration, storage fees, mooring fees, insurance, maintenance, upkeep, and other costs--the cost of the fuel consumed by a 13-foot boat is usually a small portion of the total cost of ownership.

If you reduce the horsepower of a planing hull boat by a factor of 0.5, and if the weight stays about the same, the boat speed at maximum throttle will decrease by a factor of about 0.7 compared to the other power.

[Qtrmeg]

When I was a kid I had a 1971 13' with a Mercury 20 hp on it and it was fine, as long as it was just me or a couple of us. I didn't have a speedometer so I can't tell you what the top speed was, but it was equal to my friends 13' with a Johnson 25 hp.

Back then engines were rated at the powerhead so todays engines, rated at the prop, will be more powerful at the same horsepower.

While you will probably be ok with a 20 hp engine I would recommend looking at a 25 hp engines as well. Mercury engines go to a bigger block with the 25 hp so you will pay a penalty in weight and price for that extra 5 hp. I don't know what other manufacturers do but it is worth seeing if you can get extra hp in the same block/weight.

[jimp]

davidc--growing up we had a 1963 Sport [13] with 18 HP Johnson from 1966-1981. Two 14-year-olds could slalom ski--barely: no observer in the boat, one driver, one skier, and, yeah, before we were concerned with safety. The 18-HP lasted for half a summer and then a new 33-HP Johnson that did fine. Top speed with the 18-HP was well below 27-nautical-miles-per-hour. Where does 27-nautical-miles-per-hour come from? The 13 with the 33 was "just a tad slower" than Dad's 32-ft Chris Craft Sea Skiff Sportfisherman with twin 210-HP Chris Craft 327 blocks. The Chris did 27-knots with analog knot-meter (accurate as compared to Distance/Time=Speed between known buoys). The 18 was quite a bit slower than the 33.

New 20 HP in 2015? Maybe 24-nautical-miles-per-hour with light load. But that's just a guess.

JimP

[padrefigure]

I have a 1968 Whaler 13 with Sport interior and have tested with two engines. With a 1983 Johnson 35 we ran as a bare hull, tiller steered while restoring the woodwork. With just a driver and a cooler to sit on and six gallon gas tank, we saw 29 MPH on a hand held GPS. We were running a 10 x 13 stainless steel propeller. With the interior installed, driver, one passenger, we were still able to get 26 to 27-MPH, but had a little trouble blowing out with the lower unit tilted out to the third step. We eliminated the blow out, but lost a couple miles an hour by bringing the trim in one step. We do not have a tachometer, so I am not sure what RPM the engine was running, but it seem high.

On the same boat, we swapped for a 1980 Evinrude 25-HP with a tired aluminum prop of unknown pitch. We were able to reach 19-MPH with the 25-HP engine. Again, no tachometer, but the RPM did not seem high for this engine. This was with the full interior installed.

With these benchmarks, I suspect that you will plane easily with a light load and 20-HP. Unless you are getting a very good deal on the 20-HP, I would hold out for a good 35 to 40-HP engine. But if the price is right, I think smaller-HP engines hold their value pretty well, particularly if they have tillers and are located in the upper Midwest. Put the 20-HP on it now, go have fun, and upgrade when the right next engine comes along. It is all part of the fun of having a small boat.

[PJMSport15MY1984]

David--are you sold on 20-HP engine because you have a line on a good deal? There are plenty of reasonably priced new and used outboards especially this time of year. I would put on a 25-HP engine as your minimum horsepower [for] repowering a Sport 13, and I would look at used ones before new ones. Good Luck --Paul

[arcadiainc]

I had a 13 footer with a new 20 hp Johnson back in the early 1970's. It was slow! Maybe 20-21 mph. Replaced it with an old 33 hp Johnson. The performance was drastically better. Personally I would have nothing less than 30 hp on the boat.

[moments]

I have a 1982 Sport 13 modified with a stand-up center console. [Performance] was okay with a Johnson 35-HP, but with the 1989 Johnson 48 SPL [the performance] rocks. [The Johnson 48 SPL] has a an 11.5 x 19 aluminum propeller. [The 1982 SPORT 13 boat] will take two 215-lbs adults, two six-gallon gas tanks, and a 20-gallon bait tank up and down all the southern California coast [for] fishing and diving in safety and at a speed, without the engine sounding one bit like it is straining.

A little power is a great thing in a headwind.

The center console , [fuel tanks], and battery are all slightly forward so that [the boat] sits in the water balanced on its lines.

[jimh]

Boat speed is always related to the ratio of the weight to horsepower. In a small boat like a Boston Whaler 13-footer, the total boat weight will vary greatly depending on the crew weight and the amount of gear carried, because those added weights are often greater than the hull weight. So if one plans on making long runs at planing speed carrying a very heavy load, such as two large adults and a lot of gear, it is no surprise that more horsepower will be needed to handle that total boat weight that would be necessary for a 13-footer with minimal gear aboard and being run by some skinny teenager.

I would not recommend a 34-year-old open skiff like a Boston Whaler SPORT 13 powered with a 48-HP engine as the ideal boat to use to run up and down the entire coast of Southern California carrying two large adults with a lot of fuel, bait tanks, and gear. Frankly, that sounds a bit crazy to me. Who goes out in the ocean in a 13-foot open skiff? I don't think there is a benchmark to be found in that situation regarding.