SUPER SPORT 13 with Bigfoot 9.9-HP / 15hp

Optimizing the performance of Boston Whaler boats
v12mac
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Joined: Mon Nov 12, 2018 3:27 pm

SUPER SPORT 13 with Bigfoot 9.9-HP / 15hp

Postby v12mac » Thu May 30, 2019 1:31 am

The restoration of a 1988 [SUPER SPORT 13] is finished, and the boat now has a 2004 9.9-HP BIGFOOT engine with electric start, a battery, remote steering, and a 6-gallon fuel tank. The original four-cylinder 40-HP two-stroke-power-cycle engine has been removed; it was too much power for an 11-year-old to operate.

The 9.9-HP BIGFOOT propeller is 10-3/8 x 9.5-pitch. The maximum engine speed is now too low, only 2,400-RPM, and boat speed is only 9.4-MPH. That was with with myself (220-lbs) and my son (104-lbs). With just my son the boat accelerated to 13.2-MPH, which almost calculates to a propeller SLIP = 0.

The current propeller looks like a big, flat, pusher. I believe a lot of [engine speed] is being lost.

The engine propeller shaft has 10 splines, which limits propeller choices.. I plan to get a propeller with smaller diameter.

[Discussion about carburetor changes to increase power are now in their own thread.--jimh]

Readers: give me any thoughts you have [about the proposed change in propeller diameter]?

I am out of my comfort zone in choosing a propeller for this small engine.
Last edited by v12mac on Sat Jun 08, 2019 5:14 pm, edited 1 time in total.

v12mac
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Joined: Mon Nov 12, 2018 3:27 pm

Re: SUPER SPORT 13 with 9.9-HP

Postby v12mac » Thu May 30, 2019 5:51 pm

Merc spec is 5500 wot max

Yep the 3" gearcase on the bigfoot is working against us. So is all the additional weight. But it's the motor I have.

The prop looks like 3 table spoons. Looks like a tugboat prop.

It is only 9.5" of pitch but I think it might be out of wack.

Solas makes a few props for this motor but they start at 10' of pitch.

More data Friday.

dtmackey
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Re: SUPER SPORT 13 with 9.9-HP

Postby dtmackey » Thu May 30, 2019 10:26 pm

Is the Mercury Bigfoot the 4 stroke model? If it is, I don't think you'll see much better performance since 4 stroke motors lack the down low torque of a 2 stroke to get the boat on plane and the engine to the upper RPM range where it makes power. I can tell you the difference between a 4 stroke 9.9 Mercury and a 2 stroke Yamaha 9.9 is night and day since in our cruising group I've had the chance to try many RIB dingies with various motors over the years. I can also tell you for certain that the Yamaha 2 stroke 9.9 is a simple carb change to change it to 15hp and several of my friends went this route with great results.

My suggestion would be to locate a good running 15hp 2 stroke for the kids since the 13' Whaler will run better and not be a speed demon with 15hp.

At that age, my kids had a 40 Johnson, bit I put a throttle limiter on the linkage, but my son was ultra careful and probably didn't need it.

Image

D-

jimh
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Re: SUPER SPORT 13 with 9.9-HP

Postby jimh » Fri May 31, 2019 9:28 am

v12mac wrote:The [propeller]...looks...like a tugboat [propeller].


I don't know that I can recognize a "tugboat propeller" on sight. Most tugboats have very deep draft and their propellers are far too deep in the water to be seen. And I don't know that I have ever seen a a tugboat on a graving dock with its propeller visible.

Most "tugboats" today use much more sophisticated drive systems than a single propeller on a straight shaft. I am not sure there is a genre for "tugboat propellers" now, if there ever was one.

Three-bladed propellers are by far the most common propellers, as they seem to provide the best all-round performance.

v12mac
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Re: SUPER SPORT 13 with 9.9-HP

Postby v12mac » Fri May 31, 2019 9:55 am

[The 9.9-HP engine under discussion] is the larger 323-cc or 19.7-cubic-inch block made by Yamaha. I had to go with a 9.9-HP engine for local laws. (A 40-HP four-cylinder two-stroke-power-cycle outboard engine is now in my garage.)

By tug boat prop I meant prop is designed not for speed but to push (like a slow trolling auxiliary engine) and bite in reverse. My frustration is that at 9.5-pitch and still being under recommended RPM there aren't any propeller options on the market that I can find. The 3-inch gear case and 10-spline shaft are the challenges.

I actually have a container full of propellers at work, but none are for under 200-HP applications.

jimh
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Re: SUPER SPORT 13 with 9.9-HP

Postby jimh » Fri May 31, 2019 10:16 am

A propeller pitched at 9.5-inches will have a rather flat look to the blades.

According to Boston Whaler literature (c.1998) a 9-HP engine should get a 13-footer on plane with a minimal load and a bare, dry hull. I think that is what was achieved with your boat with just your son in the boat and the 13-MPH boat speed.

v12mac
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Re: SUPER SPORT 13 with 9.9-HP

Postby v12mac » Fri May 31, 2019 10:53 pm

More data:
Engine: stock 9.9 Mercury Bigfoot; 2.42:1 gear ratio;
Propeller: 9.5-pitch propeller
Speed are in statute miles-per-hour and are speed-over-ground from GPS receiver

WITH STOCK CARBURETOR
Crew: 220-lbs Dad and 95-lbs kid
MPH = 9.4
RPM = unspecified; tachometer unreliable

As above but
Crew: kid alone
MPH = 13.2

WITH CHANGE OF CARBURETOR (to one from the 15-HP model; otherwise same conditions)

Crew: Dad and kid
MPH = 16.3
RPM = limited by Rev-limiter.
1.png
Fig. 1. A screenshot showing a "Max" of 16.3 (MPH)
1.png (4.95 KiB) Viewed 6182 times


Crew: kid alone
MPH= 19.4
RPM = limited by Rev-limiter
2.png
Fig. 2. A screenshot showing a "Max" of 19.4 (MPH)
2.png (4.95 KiB) Viewed 6182 times


[Extensively re-organized data to a more readable tabular presentation. Reduced size of giant screen captures to show only relevant information--jimh]

v12mac
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Re: SUPER SPORT 13 with 9.9-HP

Postby v12mac » Fri May 31, 2019 10:59 pm

In conclusion and support of prior suggestions:

  • if you want to run a 9.9-HP get a light, pull-start, two-stroke engine and a can of diet coke.
  • avoid the heavier super sport
  • BIGFOOT gear case robs speed
  • buy the 15hp

v12mac
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Re: SUPER SPORT 13 with 9.9-HP

Postby v12mac » Sat Jun 01, 2019 2:53 pm

I ordered a Solas three-blade with 11-pitch. I'll post data about performance after it arrives.

v12mac
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Re: SUPER SPORT 13 with 9.9-HP

Postby v12mac » Thu Jun 06, 2019 8:44 pm

This is the OEM 9.5" next to the Solas 11"

Image

jimh
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Re: SUPER SPORT 13 with 9.9-HP

Postby jimh » Fri Jun 07, 2019 9:14 am

It looks faster already, just from the picture.

v12mac
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Re: SUPER SPORT 13 with 9.9-HP

Postby v12mac » Sat Jun 08, 2019 5:00 pm

When you are 11 years old, this boat is plenty fast. I'm having a great time squeezing every bit out of this rig with my son.

New prop Solas 11-pitch; same test conditions as before

Dad and son
MPH = 21.4
RPM = 5400

Son only
MPH = 22.2
RPM = 5700

13footer.jpg
Boat is set up. Now time to make it purdy.
13footer.jpg (50.52 KiB) Viewed 5883 times

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dg22
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Re: SUPER SPORT 13 with Bigfoot 9.9-HP / 15hp

Postby dg22 » Sat Jun 08, 2019 6:43 pm

Wow! That is great for a 15 hp on a 13-footer. Good pick on the prop.

v12mac
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Re: SUPER SPORT 13 with Bigfoot 9.9-HP / 15hp

Postby v12mac » Sun Jun 09, 2019 12:14 am

I'm super happy with the performance, ran 20-MPH most of the day and did a bit of fishing.

The prop selection was 1/2 voodoo and as I shocked my son. had to use math--he liked that line.

Also l've had some great advice from lots of good folks around the web.