Outboard placement

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Bartman
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Outboard placement

Post by Bartman » Mon Jan 13, 2020 6:47 pm

I'm still up in the air between building and buying. I want a displacement or semi displacement power cat. I have been studying for sometime and curious what the thought process is between placing the outboards centered in the rear of each hull, cheating them to the center but still on the rear of the hull, and now I see several desings with twin outboard but they are mounted inside the hulls about 10-15% inward from the transom, almost in a well. I like the Supercats Sliver 29 and Sportcat 38. I also like the Gemini 399. Blue Planet is for sale....



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jacquesmm
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Re: Outboard placement

Post by jacquesmm » Wed Jan 15, 2020 10:20 am

On Richard's cats, the preferred location is on the hulls center line. He shows a single outboard between the hulls as an option but the twins work better.
Single outboards on the center line are fine for pontoon boats of for sailing cats like the Wharrams.
The offset engines are located that way to reduce draft and limit exposure to the props but there is a loss of efficiency, it's a trade-off.

PS: this was a question for Richard Woods but he is sailing right now, probably between Florida and the Bahamas.
Jacques Mertens - Designer
http://bateau.com

Bartman
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Re: Outboard placement

Post by Bartman » Wed Jan 15, 2020 11:39 am

Thank you for the reply. This is the design with twins inboard of the hulls: http://asia-marine.net/sale/brokerage/p ... -29-amch12

It does seem inefficient but it's kind of nice not having to work around the outboards.

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Re: Outboard placement

Post by Woods Designs » Mon Jan 27, 2020 12:01 pm

I use a central steerable outboard on my folding powercat designs, Skoota 20 and 24. That way there are no oil sump problems when the hulls (and thus engines) rotate for trailering. My Skoota 18 is small enough that a single outboard is enough and twin outboards mean twin fuel tanks, batteries etc so all taking up more of the very linited space

My bigger designs, Skoota 28/30, Jazz 30, Skoota 32, 36 all use an engine mounted on each transom

The problem with the central engine is that it is more prone to damage and more important, speed is limited due to the drag of the leg with no hull in front of it. In fact without a fairing, which I have designed to fit below the central nacelle, even a large outboard turns into a "food mixer" creating froth rather than forward speed at anything over about 6 knots. We tested that on our Skoota 20. Without the fairing the boat did 6 knots, with the fairing refitted it did 13+

Our Skoota 28 did over 16 knots empty using twin 20hp outboards. Loaded for cruising we did 11-12 (note the past tense, thanks to Dorian!). Crossing the Gulf Stream to the Bahamas comfortably in 5 hours. How does that compare to the similar length Thailand boat?

Richard Woods of Woods Designs

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Re: Outboard placement

Post by Woods Designs » Mon Jan 27, 2020 12:01 pm

I use a central steerable outboard on my folding powercat designs, Skoota 20 and 24. That way there are no oil sump problems when the hulls (and thus engines) rotate for trailering. My Skoota 18 is small enough that a single outboard is enough and twin outboards mean twin fuel tanks, batteries etc so all taking up more of the very linited space

My bigger designs, Skoota 28/30, Jazz 30, Skoota 32, 36 all use an engine mounted on each transom

The problem with the central engine is that it is more prone to damage and more important, speed is limited due to the drag of the leg with no hull in front of it. In fact without a fairing, which I have designed to fit below the central nacelle, even a large outboard turns into a "food mixer" creating froth rather than forward speed at anything over about 6 knots. We tested that on our Skoota 20. Without the fairing the boat did 6 knots, with the fairing refitted it did 13+

Our Skoota 28 did over 16 knots empty using twin 20hp outboards. Loaded for cruising we did 11-12 (note the past tense, thanks to Dorian!). Crossing the Gulf Stream to the Bahamas comfortably in 5 hours. How does that compare to the similar length Thailand boat?

Richard Woods of Woods Designs

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Re: Outboard placement

Post by Bartman » Tue Jan 28, 2020 2:01 pm

Woods Designs wrote:
Mon Jan 27, 2020 12:01 pm
I use a central steerable outboard on my folding powercat designs, Skoota 20 and 24. That way there are no oil sump problems when the hulls (and thus engines) rotate for trailering. My Skoota 18 is small enough that a single outboard is enough and twin outboards mean twin fuel tanks, batteries etc so all taking up more of the very linited space

My bigger designs, Skoota 28/30, Jazz 30, Skoota 32, 36 all use an engine mounted on each transom

The problem with the central engine is that it is more prone to damage and more important, speed is limited due to the drag of the leg with no hull in front of it. In fact without a fairing, which I have designed to fit below the central nacelle, even a large outboard turns into a "food mixer" creating froth rather than forward speed at anything over about 6 knots. We tested that on our Skoota 20. Without the fairing the boat did 6 knots, with the fairing refitted it did 13+

Our Skoota 28 did over 16 knots empty using twin 20hp outboards. Loaded for cruising we did 11-12 (note the past tense, thanks to Dorian!). Crossing the Gulf Stream to the Bahamas comfortably in 5 hours. How does that compare to the similar length Thailand boat?

Richard Woods of Woods Designs
Richard thanks for taking the time to reply. I think your design is more efficient than the one I referenced. The Thailand boat looks like a sailing cat built as a power cat. I did get a little performance info from the seller. It cruises low to mid teen knots based on loading. But that's at twice the horsepower of your Skoota 28 with 20's with similar speed.

Bart

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