Shanty boat build - self design BBC materials

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TomW1
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Re: Shanty boat build - self design BBC materials

Post by TomW1 »

jbo ran your shanty boat at 4500lbs again through my calculators once again. A 6HP is not feasible, only 5 mph at top speed and no remote steering on either Mercury or Yamaha motors. A 9.9HP is a little better with a top speed at 8-9mph and the possibility of remote steering. That will give you an efficient cruise of 5-6mph. In my opinion the Yamaha 9.9 with remote steering would be the best motor for you. Dependable and allow you if needed to go over higher currents and tides.

Well good luck with your build and post pictures.

Regards, Tom
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jbo_c
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Re: Shanty boat build - self design BBC materials

Post by jbo_c »

Thanks, Tom. I was thinking one of the Bigfoot motors would be a good call long term. Realistically, it will be an old 2 stroke to start with.

Jbo

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Re: Shanty boat build - self design BBC materials

Post by fallguy1000 »

TomW1 wrote: Tue Aug 10, 2021 4:05 pm jbo ran your shanty boat at 4500lbs again through my calculators once again. A 6HP is not feasible, only 5 mph at top speed and no remote steering on either Mercury or Yamaha motors. A 9.9HP is a little better with a top speed at 8-9mph and the possibility of remote steering. That will give you an efficient cruise of 5-6mph. In my opinion the Yamaha 9.9 with remote steering would be the best motor for you. Dependable and allow you if needed to go over higher currents and tides.

Well good luck with your build and post pictures.

Regards, Tom
Be careful to make sure the prop can be big enough... the big beef for me on my kicker is the prop size is limited to the shaft to cav plate dimension.
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silentneko
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Re: Shanty boat build - self design BBC materials

Post by silentneko »

But is 9.9hp enough to push 4500lbs against a light wind, say 10mph, once you take the windage into consideration? This is what you need to work out before you choose your power.

If you haven't yet, I would go to boatdesign.net and post up your questions on the forum. Many of the guys there are experienced designers and naval architects.
Built: 15ft Skiff, 16ft Skiff, Modified Cheap Canoe, and an FS17.

TomW1
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Re: Shanty boat build - self design BBC materials

Post by TomW1 »

No Silentneko a 9.9HP only gives him a max speed of 9-10mph top speed as I explained in my post. :D But it is his build and he wants it to be low powered. I forgot about wind so thanks as I was only thinking about water effects. :oops: So thanks.

Regards, Tom
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Re: Shanty boat build - self design BBC materials

Post by jbo_c »

Good points. On the plus side, at least initially, it will be fitted with an 18hp(because I own it already). I can use performance there to inform my choice.

Horsepower is not the primary consideration. I just would like to use the least that provides satisfactory performance. The 18 I have is a family “heirloom” that I have no other use for, so if it’s the right fit, that would be more than fine. However I don’t want to use it so badly that I’d spend double on fuel over what I’d get with a 9.9.

Jbo

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Re: Shanty boat build - self design BBC materials

Post by silentneko »

Keep in mind that that 18hp is very inefficient. I had a 18hp fast twin long ago, but only used it for a short time before upgrading. My 20hp yamaha 4-stroke got almost twice the mileage as the 18hp 2-stroke. Prop selection helped a bit since there's not much out there for the 18hp.

Actually my new 60hp tohatsu does pretty well despite being more then 3x the power. Cruising I think I'm getting around 12mpg with my FS17.

Just for info purposes, that little houseboat we rented had a 40hp Etec on it. It pushed the boat to a max of 6.5mph no matter the RPM, but it was climbing and tossing a huge wake. It was pretty happy at 5.5mph. We went I think 37 miles total, but only burnt maybe 3.5 gallons of fuel. Thats pretty fuel efficient to me. Not sure what your target numbers are.
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Re: Shanty boat build - self design BBC materials

Post by jbo_c »

Yes. My burn with the 18 will be about 1.5 gph based on past experience. Funny thing is, it’s pretty much the same whether at idle or 75%. Wot uses a little more.

So I don’t plan on using it long term, but it will put me on the water so I can shop for the permanent power plant at my leisure.

We’re working on our trip one of the two weeks surrounding Thanksgiving. I appreciate your turning me on to that place. I’m looking forward to it.

Jbo

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Re: Shanty boat build - self design BBC materials

Post by jbo_c »

silentneko wrote: Thu Aug 12, 2021 1:53 pm
Just for info purposes, that little houseboat we rented had a 40hp Etec on it. It pushed the boat to a max of 6.5mph no matter the RPM, but it was climbing and tossing a huge wake. It was pretty happy at 5.5mph.
Which, again, begs the question for me: Why bother putting a 40 on it? A 15 or likely even a 10 would have pushed it as well.

Jbo

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Re: Shanty boat build - self design BBC materials

Post by Jaysen »

jbo_c wrote: Fri Aug 13, 2021 7:18 am
silentneko wrote: Thu Aug 12, 2021 1:53 pm
Just for info purposes, that little houseboat we rented had a 40hp Etec on it. It pushed the boat to a max of 6.5mph no matter the RPM, but it was climbing and tossing a huge wake. It was pretty happy at 5.5mph.
Which, again, begs the question for me: Why bother putting a 40 on it? A 15 or likely even a 10 would have pushed it as well.

Jbo
Short answer: sometimes you are fighting more than just the bow hump.

The displacement hull can’t beat the hump. That limits max speed. This is “hull speed”. But what does it take to beat a 10mph wind on the nose? Now you need to beat that BEFORE the HP is limited by the hump.

This is the problem with high wind profile boats and low HP. My Helms only need a 4hp motor to make hull speed. It needs 6hp to make headway in a 25mph wind. Hence the 9.9 two smoke on her transom.
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