13-footer E-TEC 30-HP

Optimizing the performance of Boston Whaler boats
joemahoney79
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Joined: Fri May 07, 2021 1:53 pm

13-footer E-TEC 30-HP

Postby joemahoney79 » Thu Oct 14, 2021 11:40 am

I am afraid that we may have just lost the 2 stroke 40 HP Yamaha on our 1988 13' Boston Whaler. I am leaning toward replacing it with a 30 HP Evinrude E-Tec in consideration for how these boats don't handle transom weight well. What type of performance reduction should I expect? My guess is the the reliability will be well worth the trade off. The boat is used primarily by my two young boys ages 14 and 12.

dtmackey
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Joined: Thu Sep 28, 2017 9:29 pm

Re: 13-footer E-TEC 30-HP

Postby dtmackey » Fri Oct 15, 2021 10:16 am

When I was a kid 13' Whalers sold in my area came with a Johnson or Evinrude 25 or 35hp. Considering in that day and age, those motors had horsepower rated at the powerhead and not the prop and they were fine for kids ripping around. I'd estimate the 35 was closer to 30hp and never lacked for power. Will you see a reduction? Yes, but I don't feel the kids will have a problem with it.

You may notice the power difference if loading the boat to it's max capacity or towing lot of water toys.

D-

goldstem
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Joined: Thu Oct 29, 2015 12:57 pm

Re: 13-footer E-TEC 30-HP

Postby goldstem » Fri Oct 15, 2021 10:23 am

I believe you will see a performance difference, as that Yamaha 40 already used the new power rating standards. However 30hp, especially in 2 stroke form, is very good power that hull, even thought a newer Etec will weigh a bit more than the classic OMC 20-35 motor on which it is loosely based. (that OMC remains my favorite motor for the old 13).

jimh
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Location: Michigan, Lower Peninsula
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Re: 13-footer E-TEC 30-HP

Postby jimh » Fri Oct 15, 2021 6:08 pm

If the boat's power is reduced to 30-HP from 40-HP, and if the weight stays the same, then the top speed will decrease by a factor of the power ratio to the exponent 0.5, or in this case by (30/40)^0.5 or 0.86. If the prior top speed were, say, 30-MPH with the 40-HP engine, then the top speed with the with 30-HP engine would be 40 x 0.86 = 25.8-MPH.

Change in boat total weight, which is usually affected most by the weight of crew, will have a similar but opposite effect, that is, increasing weight produces reducing top speed, in the same proportion (to the ratio to the exponent 0.5).

Let us say the power reduces to 30 from 40-HP and the weight increases such that that if the total weight before were 1,000-lbs the new weight is 1,050-lbs. We then look at the power-to-weight change, or (30/40)/(1050/1000). That simplifies to 0.75/1.05 or 0.714. Taking the power to the 0.5 exponent gives 0.845. Going back to our original speed of 30-MPH, this means the speed with less power and more weight will reduce to 25.3-MPH.

A friend has a classic 13-footer that had the 15-inch notched transom. He put on an E-TEC and raised the transom height to 20-inches with some very nice fiberglass work. The boat is mainly used by his young nephews and nieces; they love it.