Over weight boat?

Bruce, I talked with one of the owners of a 31 this morning and he seams to think you are a little slow for that set up. He thinks his boat is 8 ton. He puts lobster pot loads ( up to a ton) on it all the time and doesn't have much effect on speed. Is your wheel a true 24 x 24? I think 3208s are only supposed to turn 2800. You may be hitting a governor at 2850? I guess you have plenty of things to try. Clean the bottom. Even if you aren't buying it and its going back in the water. Clorox the bottom. Make sure the wheel is clean too. If it is pitted see if the owner will allow you to take some 220 sand paper to it. Try reshifting weight toward the stern. If any of that makes a difference I'm sure more intence work on the bottom and a professional wheel reconditioning of the wheel might work wonders if you get the boat. Maybe you can even lose some of that weigth once its yours.

 

Jango. YIKES Those are the slowest speeds I've heard yet!!!! I already bought the 26 x 22 wheel so I should be able to go up and down in pitch to get the right RPM turn. For the heck of it can you run a scan for the boat I'm in now? Its 35', 200 hp volvo @ 3800 rpm with a 2.5 to 1 borg warner. Its been a sweet set up and am trying to duplicate it for the new boat. If you get the wheel size and speed right ish I suspect it will be about right for what you just gave me. tia

 

hemidodge,

My information is the wheel is a true 24x24. The spec sheet on the engine says 2900RPM. Did the owner of the other boat say what wheel he's turning? It seems the best bet is to have another prop on hand to try but that is easier said than done for a sea trial. The boat is located in Santa Barbara and I've been told bottom growth is a big problem there. The bottom paint was redone one year ago. Here in Alaska, where I live bottom growth is slower and less of a problem. The 45% slip does seem the prop is not matched correctly.

 

Bruce,

First of All, the Prop Slip is a function of the Total Load and the size and shape of the Prop trying to move it.
The Prop Pitch is the theoretical distance the prop (and boat) will move during ONE Revolution. If the prop is not sized properly - BOTH Dia and Pitch, the theoretical distance will not be achieved. Prop will tend to spin, but do little. The difference between theoretical travel and actual is the slip, expressed as a % . It should be obvious that a Bigger Prop will slip less (Grab more water) Properly made calculations take into cosideration, Boat Type (Semi - Disp) weight, HP, rpm, gearing and RPM. As previously stated the 28 x 20 Prop should have between 27 - 29% slip with your setup, which is probably the best posible with your weight and HP.

Secondly, I assume the 122 no. came from the Speed prediction section. Based on your actual speed the site gave you a Hull factor (H)of 122. This is not correct since it is based on a overly sliping Prop.i.e 14.26 mph. The Formula is as follows:

Speed (mph) = H (sq rt. [HP\ WEIGHT])

Your Hull factor is probably closer to 150, providing a top speed of abt 18 mph with the correct prop. The Hull factor takes into consideration the probable prop Slip

Forgot to mention, If your 24 x24 had a resonable slip of say 27% your speed would be 21.4 mph wich is not posible with your weight and power. 24 x 24 is not the correct prop.

 

Bill swings a 26 x 28 dyna quad behind a 225 Deere and a 2.5 to 1 gear. I slow down 5 mph on a dirty bottom. I skipped a haul out this spring figuring I'd have the new boat by mid june or 4th of July by the latest. When it was obvious it wasn't going to happen I beached the boat in mid june and cloroxed the bottom. I regained my speed temporarally but now the growth is back and am going slower than ever.

 

hemidodge, I'll need the weight in order to do a calculation

 

Jango
I don't know what my boat weighs.. I tell everyone 15000. It might be more with all the junk that collects over nthe years. tia

 

Hemidodge,

My numbers are only as accurate as the info provided. I,m not guessing.

Using 16000 lb, 2.5 gear, 200 hp @ 3800, semi displacement hull.

Recommended Prop is a 22 x16, providing 13.4k or 15.4 mph, resulting in 29% slip with a clean hull. Obviously you'll need more prop with a dirty hull and get less speed.

Another 2000lb should only cost you 1 mph

 

I suppose you are right. We can guess at my boats weight all day and still not know anything. I am slightly over wheeled now with a 22 x 22 4 blade. I repowered a few years back and this engine fit the same footprint of the old engine. The old engine was a 165 hp at 3800 rpm volvo tamd40b and swung a 22 x 20 perfectly. The new engine is a 200 hp at 3800 volvo tamd41p so I bumped up the wheel to 22 x 22 when it was time for a new one. Next time it is off its going back to 22 x 20. I think volvo just played with the numbers since the 200 hp engine is still swinging the same wheel properly. ..... They probably did the same thing with the 330 I bought. It replaced the 315. Well anyway...The boat is going 16 mph at 3400 wot dirty bottom and all. I was doing 19.5 at 3600 in June. And 21.5 @ 3800 with a 22 x20 when the machanic took it for a test ride after the engine install. If it weren't so fuel effecient Id probably paint the bottom and take all the junk out of it with these fuel prices.

 

Jango

thanks for the input. The Volvo website has a engine configurator on the pleasure inboard section and it also gave a 21 mph for a wot on the new boat. The builder thinks a possible 27 mph if I geared and wheeled it right. He talked alot about the slip thing. He wanted me to go with 3 to 1 and a 28 X ? but I didn't because it was a 6 month wait for zf to make one special for me. I was thinking about repair parts. I didn't want to wait 6 months if the worst happened. I'm commercial so that would bankrupt me.

 

John,
Thank you for educating me on the issue of prop slip and all the calculations that go into it. I'm a little confused because when we entered the parameters of what was actually achieved yesterday we came up with a 45% slip. This was based on 12.4 knots @ 2850 rpm (2.5 gear) with a 24 pitch Prop which calculates to 45% Slip.
When I run the numbers with a 28x20 prop and a slip of a more reasonable 27% . This is what I get:
I'm saying 20000# because I'm not sure how much fuel was on board during weighing which showed 19000 so I'm figuring some added fuel and water weight. Boat speed: using 20000#, a 150 constant, 260 hp, equals a theoretical speed of 17MPH.
Now using the slip calculator: gear ratio of 2.5, pitch of 20, RPM of 2850, theoretical speed of 17 MPH, and a improved slip of 27% the actual speed shows 12MPH which is 10.4 knots. This is slower than what was achieved with the 24x24 with 45% slip. What am I doing wrong? Shouldn't the boat be going faster than yesterday? Obviously the constant is the variable here.

 

3:1 Gears will work out better with 14000 lb+ boats, as I pointed out earlier in this trend. With the slower shaft speed, props will, however be Bigger. If you're able to swing a 22p at rated rpm, your boat is closer to 13000 - 14000 lb, rather than 16000.
I have never done much calculating for Semi Disp boats but always have been right on for Heavy Planning as well as High Speed Planning Hulls.
My own Pleasure Boat Is a 20' Chris Craft clone, weighing 2200 lb and powered by a 450 HP Ford sm blk, turning a 12 x 15 Prop at 6500 rpm - 87 MPH with 6% slip.

 

Bruce,
When I enter:

Gear Ratio = 2.5

Pitch = 20

RPM = 2900

Theoretical Speed is 22 (NOT 17)

enter slip of 27%

actual speed is 16mph or 13.9 knots

These no. are rounding off (In my first posting (way back when?) I gave you 13.8 Knots

Also, when using the Formula w. weight, HP, the constant already takes into consideration probable SLIP and is probably a little High (140 - 145 instead of 150) since bottom line should be 16 not 17 (actual speed)

 

Jango

87 mph??? you do mean... on a trailer with a hemi dodge towing it?

 

John,

So that is 1.5 knots better than the 45% slip 24x24 prop or a 12% improvement. Is it possible is will be even more?