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Old 14 February 2018, 06:44   #1
zip
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Speed Claims

I am researching my next used boat, and the Protector 28 Targa is on top of my list.
The used boats I've seen on Yachtworld have twin 115s up to twin 250s.
The boat weighs at least 6000 pounds.
A boat just became available with twin 115s, and the ad is stating cruising speed is 34mph, and top speed is 69mph.
I don't know how the boat is propped, but kinda thinking these numbers appear to be pretty high.
I would have guessed that cruise would be around 20, and max 30, but I'm no expert.

There is another boat listed for sale with twin 150s stating 20 knots cruise, and 40 knots wot.

One boat listed has having twin 250s upgraded to 340s, with a wot of 65, which is probably possible.

For my needs, I believe the twin 115s would be way under powered. The twin 150's might work, but if I have a choice, I would like more HP.

Is there some formula that would give me a ballpark idea of speed with just knowing the weight and horsepower?
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Old 14 February 2018, 07:19   #2
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Quote:
Originally Posted by zip View Post
I am researching my next used boat, and the Protector 28 Targa is on top of my list.
The used boats I've seen on Yachtworld have twin 115s up to twin 250s.
The boat weighs at least 6000 pounds.
A boat just became available with twin 115s, and the ad is stating cruising speed is 34mph, and top speed is 69mph.
I don't know how the boat is propped, but kinda thinking these numbers appear to be pretty high.
I would have guessed that cruise would be around 20, and max 30, but I'm no expert.

There is another boat listed for sale with twin 150s stating 20 knots cruise, and 40 knots wot.

One boat listed has having twin 250s upgraded to 340s, with a wot of 65, which is probably possible.

For my needs, I believe the twin 115s would be way under powered. The twin 150's might work, but if I have a choice, I would like more HP.

Is there some formula that would give me a ballpark idea of speed with just knowing the weight and horsepower?
69mph with twin 115's is going some................ In the real world your probably in the 30-40mph range
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Old 14 February 2018, 08:09   #3
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Quote:
Originally Posted by zip View Post
I am researching my next used boat, and the Protector 28 Targa is on top of my list.
The used boats I've seen on Yachtworld have twin 115s up to twin 250s.
The boat weighs at least 6000 pounds.
A boat just became available with twin 115s, and the ad is stating cruising speed is 34mph, and top speed is 69mph.
I don't know how the boat is propped, but kinda thinking these numbers appear to be pretty high.
I would have guessed that cruise would be around 20, and max 30, but I'm no expert.

There is another boat listed for sale with twin 150s stating 20 knots cruise, and 40 knots wot.

One boat listed has having twin 250s upgraded to 340s, with a wot of 65, which is probably possible.

For my needs, I believe the twin 115s would be way under powered. The twin 150's might work, but if I have a choice, I would like more HP.

Is there some formula that would give me a ballpark idea of speed with just knowing the weight and horsepower?
He's probably quoting numbers off the extremely inaccurate factory speedometer.

A RIB that heavy, with wind drag, is going to feel like a slug with twin F115s. Probably around 40 mph top speed.
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Old 14 February 2018, 08:59   #4
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Quote:
Originally Posted by zip View Post
I am researching my next used boat, and the Protector 28 Targa is on top of my list.

The used boats I've seen on Yachtworld have twin 115s up to twin 250s.

The boat weighs at least 6000 pounds.

A boat just became available with twin 115s, and the ad is stating cruising speed is 34mph, and top speed is 69mph.

I don't know how the boat is propped, but kinda thinking these numbers appear to be pretty high.

I would have guessed that cruise would be around 20, and max 30, but I'm no expert.



There is another boat listed for sale with twin 150s stating 20 knots cruise, and 40 knots wot.



One boat listed has having twin 250s upgraded to 340s, with a wot of 65, which is probably possible.



For my needs, I believe the twin 115s would be way under powered. The twin 150's might work, but if I have a choice, I would like more HP.



Is there some formula that would give me a ballpark idea of speed with just knowing the weight and horsepower?


Just to give a comparison, my 6.4m Ribcraft with twin 115 Etecs swinging 19" mercury rev4 props topped out at 45kn & cruised at 25kn. She was a heavy boat but nowhere near as big a lump as a protector. Factor in the cabin & extra weight & I think the seller has been on the wacky baccy.
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Old 14 February 2018, 14:59   #5
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....Is there some formula that would give me a ballpark idea of speed with just knowing the weight and horsepower?
Yes there is. I used it when designing my boat but some of the inputs will still be educated guesses since you don't have every piece of information required for absolute accuracy. It did prove to give a reasonable estimation. IIRC, hp + displacement + hull type were the primary inputs.

However, I can't point you directly to it but it was on one of the more serious USA boating sites.

The 69mph on twin 115s is plainly silly.
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Old 15 February 2018, 04:48   #6
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Yes there is. I used it when designing my boat but some of the inputs will still be educated guesses since you don't have every piece of information required for absolute accuracy. It did prove to give a reasonable estimation. IIRC, hp + displacement + hull type were the primary inputs.

However, I can't point you directly to it but it was on one of the more serious USA boating sites.

The 69mph on twin 115s is plainly silly.
Rubbish, 69mph on 115's is perfectly achievable, you just need a few................
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Old 15 February 2018, 06:43   #7
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We run a Protector 28. Used to have 2x 150's and would top out at 39knts now has 2x 250's and with mid-range props tops out at 52Knts. 2x115's struggles to achieve 32Knts
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Old 16 February 2018, 07:44   #8
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Yep, that pretty much confirms. Thanks.
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Old 16 February 2018, 19:12   #9
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69 mph is easily attainable with that rig. But only on a trailer behind a big diesel truck!!!!
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Old 17 February 2018, 01:57   #10
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69 mph is easily attainable with that rig. But only on a trailer behind a big diesel truck!!!!
You laugh, but I've seen 60mph "max speed" on my GPS and it took a minute before the "penny dropped"
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