Key West engine-question

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gostap
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Key West engine-question

Postby gostap » Thu Jun 15, 2017 11:21 pm

Hello;

I've posted this earlier in design-forum..maybe it should be here instead?

Hello everybody

I'm new at the forum, as well a newbie to boatbuilding even if I have experience of professional carpeting..living in Sweden, so excuse my English.. :)
I have some question about "Key West" which is the boat I start building. It depends the engine and gear ratio. I have a GM V8 engine I intend to use in the boat. (inboard installation) After the recovering of the engine, I think there will be about 270-300hp. The boat is meant to be "loaded" with 6-7 persons and some equipment for fishing. My intention is to have a moderate marsh speed about 25-28knots and 35knots top speed.
I got suddenly a good opportunity to buy a Borg Warner valve drive (type AS7-71C) that have a gear ratio 1,91:1

Could that ratio work for the Key West?

Hope for answer from all experienced people on forum..

Best Regards;

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Roberta
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Re: Key West engine-question

Postby Roberta » Fri Jun 16, 2017 5:56 am

I think you would be OK with the reduction velvet drive transmission. I would run the HP of the motor and the specs of the boat along with the transmission ratio past someone or something that could calculate the correct prop for your desired performance. That way you will know the size of the prop needed and can order an appropriate shaft and strut. Gear reduction transmissions generally will need a bigger prop with that much HP. I would also look into the price of new 71C transmissions and compare the price. Most boats in that size range are running 1.23:1 ratios.

See if Ken S. chimes in with his thoughts.

Roberta
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

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gostap
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Re: Key West engine-question

Postby gostap » Fri Jun 16, 2017 2:44 pm

Thanks for you answer.... As I wrote, I have a good opportunity to buy this BW for about 500-600 US. I expect, that a new one will cost about 2000-3500US? But of course, I don't know if the gear ration will work on Key West?

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kens
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Re: Key West engine-question

Postby kens » Fri Jun 16, 2017 4:48 pm

That is too much gear for a V8, and that boat.
Use a 1.5:1 as the deepest gear ratio.

I say that because I am running that transmission with a 4-cylinder in a Double Eagle, and it runs fine.
IF, I upgraded to a V8, and kept that gear, I would also upgrade to 1 3/8" shaft, and turn a 18" or 20" diameter prop. It would be silly.
My current plans to upgrade is a 6-cylinder with 1.5:1

I have witnessed a steel hulled 35 footer get up on plane with a single Chrysler V8 turning 2:1.
I have witnessed a steel hulled 45 footer get up on plane with twin Chryslers turning 2:1

We are not that big, and we are not steel, we do not need that much gear.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

rblevens
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Re: Key West engine-question

Postby rblevens » Mon Aug 07, 2017 8:01 am

Could one run a 1:1 ratio with a 454 Mercruiser in the Key West?
Thanks

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Bill Edmundson
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Re: Key West engine-question

Postby Bill Edmundson » Mon Aug 07, 2017 9:32 am

There are so many parts to this question. How much HP? How much will the running weight be? What dia. prop are you planning? What is the WOT RPM of the engine? All of this goes into gear ratio, shaft dia., blade pitch, strut, and shaft log. And which gear design you will need?

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
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rblevens
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Re: Key West engine-question

Postby rblevens » Mon Aug 07, 2017 4:23 pm

[quote="kens"]That is too much gear for a V8, and that boat.
Use a 1.5:1 as the deepest gear ratio.

What would your guess be on a prop with that 454 using a 1.5:1 (Key West)?

I was going to use a 454 and 1:1 on the (Monte Carlo) boat.

Thanks!
Rick

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kens
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Re: Key West engine-question

Postby kens » Mon Aug 07, 2017 5:15 pm

A 454 BBC with a 1.5:1 sounds to me like either too much engine , or, too much gear.
Considering the Key West is similar in size to my boat, I am thinking as such:
454 BBC with 1:1,
or,
1.5:1 gear with less engine
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

Ozzieboat
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Re: Key West engine-question

Postby Ozzieboat » Tue Aug 08, 2017 3:20 am

To quote from the "Velvet Drive Installation Manual", "One hundred revolutions per minute of the prop shaft for each mile per hour of boat speed is considered a very good rule of thumb for selecting drive ratio" There is some good info in the manual, do a search for Velvet drive installation manual and one of the responses is "Velvet drive installation manual - kp44.org, copied by Tom King". It is under the Chris-Craft publications logo. He states it is copied for the benefit of classic boat owners, I think we fit.

rblevens
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Re: Key West engine-question

Postby rblevens » Tue Aug 08, 2017 4:01 am

Ozzieboat wrote:To quote from the "Velvet Drive Installation Manual", "One hundred revolutions per minute of the prop shaft for each mile per hour of boat speed is considered a very good rule of thumb for selecting drive ratio" There is some good info in the manual, do a search for Velvet drive installation manual and one of the responses is "Velvet drive installation manual - kp44.org, copied by Tom King". It is under the Chris-Craft publications logo. He states it is copied for the benefit of classic boat owners, I think we fit.

Thanks- I will!


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