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Maximum hull speed |
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cisom 
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Joined: April-23-2004 Location: United States Status: Offline Points: 21 |
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 Topic: Maximum hull speedPosted: May-11-2004 at 1:56pm |
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I have a theoretical question. I've got a '75 Southwind 20 with the 351W. What is the maximum hull speed that can be rung out of this boat? With no trim capabilities, unless a set of tabs were installed, I would imagine there is a point where no matter how much hp is available the boat won't go any faster. I know there are formulas to determine this number, but does anyone have any first hand knowledge?
This particular boat starts porposing at about 40 mph on smooth water, other than that it is very well behaved. I'm not in Afganistan. I have no idea how that got in my profile. |
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reidp
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Joined: December-06-2003 Location: Mooresville, NC Status: Offline Points: 1804 |
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Posted: May-12-2004 at 8:31pm |
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I was hoping Bill (SS 201) might add to this. From what I've seen with inboards of this type, but mostly with the more or less flat bottom models like Mustangs and Nautiques, is that they certainly will run faster. I'm sure this is the case also with the vee bottom Southwinds. But I do feel that there may be some diminishing return as it indeed takes more and more power to get each little bit of speed with the bow trying to typically bury itself. But if yours has a porpoising tendency(sp) then at least you know it's getting the bow up, whereas it's much easier to pull a bow down than lift one up, as with the tabs as you mentioned. From what I've seen, and this of course varies with boat weight, you can expect at least a 1 mph increase with about every 10 horsepower you gain. And of course that's only a rule of thumb. I picked up +/- 10-12 mph with an approx. 75 hp gain, but on a 16.5' boat. I believe 55-60 from that boat is easily realistic without going overboard on the engine, and you may indeed need those tabs if you don't want to simulate a Jersey Speed Skiff.
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Rick
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Joined: March-03-2004 Location: United States Status: Offline Points: 338 |
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Posted: May-12-2004 at 9:53pm |
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I would guess high 60 low 70 with the right prop. I think it would be a scarey ride though skipping accross the water
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77stang
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Joined: May-19-2004 Location: United States Status: Offline Points: 342 |
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Posted: May-19-2004 at 1:56am |
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lets work through a little.
I would think your question stems from you wondering "why your porposing at 40 mph" and if thats normal. Just out of couriosity, whats your speed and rpm right before the porposing begins? Also, what prop are your turning? lets start here to determine how efficient your hull is at turning pitch into mph. |
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cisom
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Joined: April-23-2004 Location: United States Status: Offline Points: 21 |
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Posted: May-20-2004 at 2:36pm |
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Speed is hard to calculate because speedo doesn't work, but it's basically WOT. Prop is 13x14RH, RPM approx 4200. Next time I get out I'll bring a GPS to figure speed.
Actually my question was posed because I have access to a very strong 460 Ford out of my dad's motorhome and a set of Nickson log manifolds. 
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64 Skier
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Joined: February-08-2004 Location: United States Status: Offline Points: 415 |
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Posted: May-20-2004 at 2:44pm |
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I've got two almost identical boats...a '71 and a '64. The '71 will porpoise...the '64 doesn't.
What typically causes this? |
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reidp
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Joined: December-06-2003 Location: Mooresville, NC Status: Offline Points: 1804 |
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Posted: May-21-2004 at 2:14pm |
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Porpoising? It was beyond me other than the typical loading and trim causes, so to get an idea as to what the dynamics involved were I went to that English scholar "Jeeves" and found:
"Porpoising: A constant rhythmic longitudinal pitching action of the boat, caused by a dynamic longitudinal instability. Often results from improper dynamic balance of weight and aero/hydro-dynamic forces, and usually occurs at specific speed for a unique hull setup. Over trimming the engine might cause this, whereby the bow is being held up by prop thrust, but not enough to stabilize the condition. Trimming 'in' can eliminate it at low speeds. The same action from the boat caused by not enough hull lift and subsequent falling of the bow back into the water. On some particular rigs and setups, this could be a transition zone from hydrodynamic lift to aerodynamic lift that needs to be driven through in some manner before the hull stabilizes. Can also be caused by a rocker." There was also some neat stuff I found from the Boston Whaler Forum, and those guys know porpoising. I simply ran a query "What causes a boat to porpoise?" and got a ton of responses, and some that may be a little easier to understand than the one above. |
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