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Old 06-04-2010, 03:58   #1
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What Speed Can I Expect from 2x20hp Engines ?

Can anybody tell me what speed can I expect from a 35 to 40ft cat under power with 2x 20hp engines.
ie against wind and current
ps Cats like simpson 35,Fountaine pajot 35,Maxim 38.
I'm thinking of trading up from my Roberts 36 mono hull that cn get hull speed of 7.5knots and at least 5 against wind and current.(good for getting out of troubles way)
Laurie
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Old 06-04-2010, 06:03   #2
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I suspect the real answer depends upon hull speed, weight & props of course.

By way of rough comparison I have 2x30hp pushing 13,500lbs displacement via 3-blade folding props and top out at 7.5 kts. This is about 1/2 knot slower than with the Autoprops that came with the boat - but they were trashed and I refuse to pay that much for pops. 5 kts into a 2+ knot current in our channel can be done no problem.

Good luck with your search.
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Old 06-04-2010, 14:21   #3
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With a pair of 20 hp Honda outboard on my 44 foot cat I can motor at a little over 10 knots flat out, and cruise at 8 - 8.5 knots with both motors, 6 knots with one.
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Old 06-04-2010, 15:47   #4
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The only inboard powered 35' Fountaine Pajot I know of is the old Tobago. If not loaded down with cruising gear, twin 20HP motors would push these boats very well because they displace less than 10,000 pounds. A Maxim 38 displaces about 13,000 pounds. With twin 20s I would expect either one to be faster under power than a typical Bruce Roberts 36. Don't know about Simpson 35s.
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Old 06-04-2010, 18:30   #5
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I would think they would be 6-8 metric tones, so:

2x8=16 hp in flat water,

upwind and current say 5 x8=40

hence you will be able to reach the hull's speed.

Just remember it may be 8 over the water and much less over the ground ;-)

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Old 10-04-2010, 23:26   #6
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Akarua

Thank a lor fella's it give me the Idea that 20hp is ok, it's just that I have a mate with a Simpson 35ft 5000kg cat that has 2x20hp engines and against the current and 15-20 knot wind off Mackay Queensland can only reach 2knots over ground, not sure if he has the right props on thow.In the same conditions my Roberts does 5 at 2500 rpm on my 39hp yanmah.sp she's a very heavy Roberts registered at 14.9 tonns.
Thanks again.
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Old 11-04-2010, 13:39   #7
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Akarua - and how much is the current? ;-)

For you see if the current runs fast enough, you can be actually moving backwards!

All measurements against the water.

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Old 11-04-2010, 15:19   #8
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we have a single 40 hp four stroke big foot mercury on our 36ft tri and at 2800 revs we cruise at 5.5 knots the fastest we have gone was 9.5 knots but we had a tail wind and were racing to catch the last raising of a bridge !
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Old 11-04-2010, 15:50   #9
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Barnakiel

The current off Mackay runs at2 to 3.5 knots
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Old 11-04-2010, 19:57   #10
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Most likely, the problem that your friend was having was not related to the power that the engines were capable of (although if the boat was pitching badly, it could) but rather not being able to utilize the power. With outboards, the propeller tends to be very close to the surface which is a huge disadvantage, especially when it gets rough. In addition, if the propeller has too much pitch for the gearing, the engine will not be operating up in the powerband and will have reduced power. Could he reach max rpm? If he could, pitch and gearing were not the issue but rather prop size/depth were likely the problem. It could always be that the boat itself was struggling but that seems unlikely.
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Old 11-04-2010, 20:23   #11
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On my Privilege 39, I have two Yanmar 28 hp engines with twelve inch fixed props propelling a boat that weighs in at 20,000 lbs fully loaded with cruising gear.

I have been able to make 3 to 4 knots to windward in a thirty-eight knot headwind on two occasions. On each occasion, I had both engines going full throttle.

When I am not going into headwinds, I can motor with two engines at 7.5 to 8 knots. When I use a single engine, I can motor at five knots in calm water.

I suspect that I could motor faster if my boat was several thousand pounds lighter. I've thought about throwing all my wife's stuff overboard to lighten ship and go faster, but decided that would be unwise.
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Old 11-04-2010, 21:27   #12
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Akarua

Thats what I thought a boat should be capable of.
Thanks
Quote:
Originally Posted by maxingout View Post
On my Privilege 39, I have two Yanmar 28 hp engines with twelve inch fixed props propelling a boat that weighs in at 20,000 lbs fully loaded with cruising gear.

I have been able to make 3 to 4 knots to windward in a thirty-eight knot headwind on two occasions. On each occasion, I had both engines going full throttle.

When I am not going into headwinds, I can motor with two engines at 7.5 to 8 knots. When I use a single engine, I can motor at five knots in calm water.

I suspect that I could motor faster if my boat was several thousand pounds lighter. I've thought about throwing all my wife's stuff overboard to lighten ship and go faster, but decided that would be unwise.
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Old 11-04-2010, 21:36   #13
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Quote:
Originally Posted by Akarua View Post
I suspect that I could motor faster if my boat was several thousand pounds lighter. I've thought about throwing all my wife's stuff overboard to lighten ship and go faster, but decided that would be unwise.
Yes indeed. Most unwise from so many perspectives. LOL

The only way that would be survivable is to throw the wife over as well. And then there are all those relatives, vendettas and police to deal with.

On the plus side, by the time she threw all your 'junk' away you could have a very fast ship.
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