2006 ALERT 15 Mercury 115-HP Pro XS

Optimizing the performance of Boston Whaler boats
EdwardSchnog
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Joined: Tue Mar 14, 2017 8:24 pm

2006 ALERT 15 Mercury 115-HP Pro XS

Postby EdwardSchnog » Tue Mar 14, 2017 8:34 pm

[I am] thinking about installing a Mercury 115-HP Pro XS on my 2006 ALERT 15 (Commercial and Government Product hull). The boat's previous engine was a Yamaha 90-HP two-stroke-power-cycle engine weighing 120-Kgs [264-lbs]. The boat had a top speed of 43-MPH at 5,500-RPM. The Mercury engine weighs 165-Kgs [363-lbs].

[I am] not sure what the expected boat speed would be, nor if the boat performance will be great. Has someone installed [a Mercury 115-HP Pro XS engine] on a classic 15 SUPER SPORT or similar ALERT model?

What can I expect?

jimh
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Location: Michigan, Lower Peninsula
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Re: 2006 ALERT 15 Mercury 115-HP Pro XS

Postby jimh » Wed Mar 15, 2017 5:27 am

Boat speed change of a moderate planing hull will be in proportion to the horsepower-to-weight ratio change to the 0.5-exponent. If boat speed was 43-MPH with 90-HP, then increasing horsepower to 115-HP and keeping the weight the same would suggest the boat speed would increase to

43-MPH x (115/90)^0.5 = 48.6-MPH

However, the weight increase of 100-lbs will reduce that somewhat. To estimate how much the added weight will affect the speed we need to know the total boat weight with the 90-HP engine. This can only be estimated, as no data is given. We can estimate the boat weight from the performance data using Crouch's method and assuming a hull factor of 180. This suggests the boat weight was 1,577-lbs. If we add 100-lbs and increase HP to 115, the boat speed estimate with Crouch's method then becomes 47-MPH.

While the potential top boat speed will increase, some consideration should be given to the effect adding 100-lbs of engine weight to the transom will have on the boat's general characteristics. Certainly the static trim of the boat will be down by the stern due to the added weight. The ability to get the boat to rapidly accelerate onto plane may also be affected by the added stern weight.

EdwardSchnog
Posts: 2
Joined: Tue Mar 14, 2017 8:24 pm

Re: 2006 ALERT 15 Mercury 115-HP Pro XS

Postby EdwardSchnog » Wed Mar 15, 2017 8:37 pm

Hi Jim--thanks for the feedback. Boat weight dry without engine is approx 750-lbs, the engine is another 365-lbs. I was hoping that the boat with engine combo would reach speeds just over 50-MPH. I ordered the engine today, so after installation I will be able to experience the performance hands-on.

Regards,
Edward

jimh
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Location: Michigan, Lower Peninsula
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Re: 2006 ALERT 15 Mercury 115-HP Pro XS

Postby jimh » Thu Mar 16, 2017 3:00 pm

Based on the Crouch method of prediction of boat speed, if your 90-HP pushed the boat to 43-MPH, to reach 50-MPH you'd calculate the horsepower as follows:

Crouch's premise:

[ (P2/P1)^0.5 x V1 ] = V2

Solving for P2

P2 = [V2/V1]^2 x P1

Solving at V1=43, V2 =50, and P1 = 90 to find horsepower needed on your boat to hit 50-MPH (based on present performance at 43-MPH)

P2 = 122-HP, and that is with no weight increase. If weight increases, more horsepower will be needed, probably about 130-HP.