13 GLS and 9.9-HP

Optimizing the performance of Boston Whaler boats
ByramCoveBrian
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13 GLS and 9.9-HP

Postby ByramCoveBrian » Fri Jan 29, 2016 1:12 pm

I have a 1989 13-foot Boston Whaler [later identified as a 13 GLS] with side console and throttle. It is currently powered by a 40-HP four-stroke-cycle engine. Our state law permits a 13-year-old to drive a boat powered by an engine of less than 10-HP. My daughter is about to turn 13. I want to re-power the Whaler with a 9.9 HP four-stroke-cycle engine. I appreciate it will have limited performance. We live on a lake, and it would be pretty cool for her and my two other kids (when they are of age) to safely putt-around in the boat. I believe the life lessons associated with it would be great.

Does anyone have any experience with [the performance of a Boston Whaler 13 GLS powered with a 9.9-HP engine]?

If I purchase a 9.9 HP long shaft engine, where would be my best bet to purchase?

Should I purchase new since I will have it for the next seven years?

Any advice is appreciated. Thank you for all your help. Brian

1989BW.jpg
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Etchase
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Re: 9.9 HP on a 13

Postby Etchase » Fri Jan 29, 2016 5:11 pm

I have a 10-HP auxiliary engine on a Boston Whaler 22-foot hull, a TEMPTATION 2200. It glides around nicely at 5 to 6-nautical-miles-per-hour. A [13-footer] will hit hull speed [with] the 10-HP.

jimh
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Re: 13-footer and 9.9-HP

Postby jimh » Sat Jan 30, 2016 7:57 am

I am not sure what speed in MPH was intended to be suggested by the comment above as the "hull speed" of a 13-foot boat. The usual definition of hull speed is:

Hull speed or displacement speed is the speed at which the wavelength of the boat's bow wave (in displacement mode) is equal to the boat length.

Source: https://en.wikipedia.org/wiki/Hull_speed

The usual formula for hull speed (also given in the above cited reference) is:

Speed (nautical-miles-per-hour) = 1.34 x Waterline^0.5 where Waterline is in feet

If we allow the waterline of a 13-footer to be a generous 13-feet, the hull speed would then be calculated to be 4.6-nautical-miles-per-hour.

I think you can make an inference from the observation provided (above) that a 10-HP engine will push a 22-foot boat to 6-nautical-miles-per-hour. If 10-HP can push a much heavier 22-footer to 6-nautical-miles-per-hour, it should have no trouble pushing a 13-footer to its hull speed of 4.6-nautical-miles-per-hour.

Converting that to the more commonly used measurement of boat speed on inland lakes in miles-per-hour gives 5.3-MPH as the hull speed of a 13-footer. I suspect that a 13-footer can probably be pushed by a 10-HP engine to a speed greater than 5.3-MPH.

jimh
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Re: 13-footer and 9.9-HP

Postby jimh » Sat Jan 30, 2016 8:17 am

At one time in c.2004 to c.2009 (and perhaps still happening), a Disney theme park in the Orlando, Florida area had a fleet (of perhaps 40 or more) Boston Whaler 17-foot MONTAUK boats that were powered by small engines, sometimes reported to be 9.9-HP and other times reported to be perhaps 15-HP. These boats could be rented by visitors and driven around the park's waterways. I suspect that those boat were probably not able to get on plane, and particularly because they were probably only rented to adults, who would tend to weigh more than a 13-year-old child.


A [13 GLS] with a 9.9-HP and with only a small 13-year-old aboard might possibly get close to or might even get briefly onto plane. You'd have to select the propeller pitch with an eye to what sort of performance you wanted to achieve.

jimh
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Re: 13-footer and 9.9-HP

Postby jimh » Sat Jan 30, 2016 8:25 am

If you change the 40-HP to a 9.9-HP engine you can simplify the rigging and save weight if you get a pull-start engine and an engine that can run without an attached 12-Volt storage battery. You have to pick the engine carefully because many modern engines cannot run without a battery attached because they need an external source of electrical power to get started and stay running at low speeds.

Some 9.9-HP outboard models will have all the trappings (and all the weight associated with those trappings), such as electric start, power trim and tilt, and a hefty alternator to provide battery charging. You can save weight, money, and complexity of rigging if you just get a pull-start tiller-steering, non-remote control 9.9-HP engine. The saved weight will improve performance. Getting a no-frills 9.9-HP will also save money.

There is also something to be said for pull-starting a small engine: it tends to reinforce the notion of keeping the engine in good repair, in good tune, and learning how to operate it properly. These are good things to teach young power boaters. Learning to operate a small boat with a tiller-steered outboard is also a good skill for a beginning boater to have.

jimh
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Re: 13-footer and 9.9-HP

Postby jimh » Sat Jan 30, 2016 8:50 am

As for buying a new outboard engine in preference to a used model, there is nothing to dislike about a new engine other than the higher cost of purchase. Depreciation of a new outboard engine's resale value is probably greatest in the first year or two of ownership and declines more slowly after that.

If you decide to go full-boat or full-bore or full-blown and get electric start, power trim and tilt, remote throttle and shift, and battery charging, you will probably want to stay with the Brunswick Mercury-brand outboard engines to avoid having to completely re-rig the boat. Mercury outboard engines are often sold on the basis of their lower selling price, and this results in much competition among dealers to cut prices on Mercury engines.

A small engine like a 9.9-HP can be shipped without too much fuss and bother. I don't think it would qualify for UPS delivery, but you won't need a pallet and fork lift to unload it from a delivery truck. This can widen your range of possible sources for a retailer of Mercury-brand engines. There is some cutthroat competition among Mercury dealers on price, with one obscure dealer in the back woods of Tennessee or Kentucky often cited as the least expensive source of almost anything with a Mercury brand on it.

On the other hand, if you do buy from out of town you will probably be expected to install the engine yourself and rig it yourself. If you don't plan on doing that work yourself, a local dealer is preferable as the selling dealer, as otherwise you might get a bit of a put-off from a dealer if you ask them to rig an outboard you bought out of town at a big price break. And you might get an expensive quote for the work, too.

ByramCoveBrian
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Re: 13-footer and 9.9-HP

Postby ByramCoveBrian » Sun Jan 31, 2016 8:44 am

Thank you for the insight. I will most likely make the new motor investment with all the bells and whistles. I have three kids that will be getting this boat. I will search up by the lake for a local Mercury dealer and see what kind of numbers they come back with. Thanks again. I hope to be fully operational by April or May. --B

Whal
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Re: 13-footer and 9.9-HP

Postby Whal » Mon Feb 01, 2016 1:56 pm

I don't know how much this will help. I have a 14-foot aluminum V-hull. Sometimes I put my 6-HP Tohatsu kicker on the boat, and the boat tops out at an amazing 10.4-MPH.

dgoodhue
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Re: 13 GLS and 9.9-HP

Postby dgoodhue » Mon Feb 01, 2016 6:32 pm

You technically have a Boston Whaler 13 GLS Boston Whaler. The "130" designator is used for the new 130 hull, which is different. The GLS is heavier than the SPORT as well. The minimum power that will [give] satisfactory performance with a light load is 9-HP. I have been in 13-foot tiller-steered boats with 15-HP, and they [are] fine, [reaching] just over 20-MPH. Maybe someone can chime in with this combination.

I would guess [maximum boat speed will be in the] high teens, but speed is going to be really sensitive to weight. The heavier interior and 9.9-HP might make this a boat [that can reach plane with no more than one adult aboard]. The outboard engine is going to be running at or near wide-open throttle a lot of the time. The usage of your Whaler maybe really limited or may become the kid's boat.

jimh
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Re: 13 GLS and 9.9-HP

Postby jimh » Tue Feb 02, 2016 8:24 am

Thanks for the proper identification of the model of Boston Whaler.