performance calculation

[whitepointer23]

hi everyone. can anyone on the forum do a calculation for me. moderate v 26 foot planing cruiser approximatley 3.8 metric tons, twin diesel 75 hp engines 1-1 borgwarners, prop size unknown. i am after a rough idea of prop sizes i need and what speed to expect. i am hoping for about 14 knots.

[daiquiri]

Is it a production boat? If it is, the best thing would be to contact the manufacturer. You can get some blind guesses here, but each will be different from another because based on assumptions, and will cost you $$ if wrong.
Cheers!

[whitepointer23]

Originally Posted by daiquiri

Is it a production boat? If it is, the best thing would be to contact the manufacturer. You can get some blind guesses here, but each will be different from another because based on assumptions, and will cost you $$ if wrong.
Cheers!

thanks for the reply. yes it is a production boat but the factory is not in business any more. i am just looking for a rough idea on what to expect. there might even be someone on the forum with a similar spec boat.

[daiquiri]

Ok, I see. Then how about posting some more data, like model name, waterline length and beam, is 3.8 tons full or empty weight, how big fuel and water tanks are, how many persons on board ets? Does it have inboard or outboard engines? What props does it have now and what speed do you get with them?

[CDK]

Sorry to disappoint you Whitepointer23, 14 kt is no planing speed for a 26 ft hull.
With twin 75 hp you can achieve 14 kt as a top speed, still in displacement and with almost 60 ltr/hour fuel consumption,

To reach stable planing speed at 22 knots you need an additional 50 hp to get over the hump, then you can throttle back to approx. 80% power.

And your boat should go on a diet, with 3.8 tonnes it is overweight.

[whitepointer23]

Originally Posted by CDK

Sorry to disappoint you Whitepointer23, 14 kt is no planing speed for a 26 ft hull.
With twin 75 hp you can achieve 14 kt as a top speed, still in displacement and with almost 60 ltr/hour fuel consumption,

To reach stable planing speed at 22 knots you need an additional 50 hp to get over the hump, then you can throttle back to approx. 80% power.

And your boat should go on a diet, with 3.8 tonnes it is overweight.

thanks for the reply. the weight is an estimate, i wanted to make it higher rather than understate it. i don't agree with 22 knots being planing speed, this style of boat planes at 11 to 12 knots, 10 with the tabs right down, you may be thinking of stern drives with the weight in the back. i have also been told that it won't cruise at 8 knots, it does that easily, the inboard lay out is very well balanced.

[whitepointer23]

i had another boat that was a similar design but 24 ft long, i removed a stern drive from that one and made a prop tunnel and fitted a 453 detroit 165 hp, it had a top speed of 22 knots and cruised at 14 knots on 11 liters per hour, that is why i question what you are saying. the speeds were on gps not guess work, the boat weighed 2.7 metric ton and the fuel was measured by filling tank after each trip and hour meter readings and trip time off gps. this boat was harder to get on the plane due to loss of bouyancy with the tunnel.

[daiquiri]

It sounds to me like your boat's specifications are similar to Tom Lathorpe's Bluejacket: http://www.bluejacketboats.com/bluejacket_25_5.htm
Though 3.8 tons is a bit higher figure, the other numbers fit well (imho) into Bluejacket's range.

Tom is a member of this forum ( http://www.boatdesign.net/forums/profile/tom28571.html ) and you can contact him directly and ask for his opinion. He is a polite and experienced person, and knows his boats well. I'm sure he'll be able to give a close estimate to numbers you're looking for.

Cheers

[CDK]

Originally Posted by whitepointer23

thanks for the reply. the weight is an estimate, i wanted to make it higher rather than understate it. i don't agree with 22 knots being planing speed, this style of boat planes at 11 to 12 knots, 10 with the tabs right down, you may be thinking of stern drives with the weight in the back. i have also been told that it won't cruise at 8 knots, it does that easily, the inboard lay out is very well balanced.

I didn't think of stern drives, just the weight to hull surface ratio.
My boat has roughly the same length, weight and horse power. The hull has been extended to 26 ft., at 14 knots it needs full throttle. Once it had more powerful gas engines and could plane at speeds between 22 and 32 knots.
The attached graph shows the calculated engine torque vs. speed. Where the torque drops, planing speed is achieved.

[whitepointer23]

Originally Posted by daiquiri

It sounds to me like your boat's specifications are similar to Tom Lathorpe's Bluejacket: http://www.bluejacketboats.com/bluejacket_25_5.htm
Though 3.8 tons is a bit higher figure, the other numbers fit well (imho) into Bluejacket's range.

Tom is a member of this forum ( http://www.boatdesign.net/forums/profile/tom28571.html ) and you can contact him directly and ask for his opinion. He is a polite and experienced person, and knows his boats well. I'm sure he'll be able to give a close estimate to numbers you're looking for.

Cheers

thanks for the link, nice looking designs, i will have a good look tomorrow.

[whitepointer23]

Originally Posted by CDK

I didn't think of stern drives, just the weight to hull surface ratio.
My boat has roughly the same length, weight and horse power. The hull has been extended to 26 ft., at 14 knots it needs full throttle. Once it had more powerful gas engines and could plane at speeds between 22 and 32 knots.
The attached graph shows the calculated engine torque vs. speed. Where the torque drops, planing speed is achieved.

i looked at the graph and can see what you mean, but it is different to what i have found in my boats. next time i slip the boat i will send you photos of the hull. the v is quite moderate as was the other boat i mentioned , i think this could be the reason we have different results. i don't know how many degrees deadrise i have but if you compare it to a bertram 25 it has a lot less v.

[whitepointer23]

Originally Posted by CDK

Sorry to disappoint you Whitepointer23, 14 kt is no planing speed for a 26 ft hull.
With twin 75 hp you can achieve 14 kt as a top speed, still in displacement and with almost 60 ltr/hour fuel consumption,

To reach stable planing speed at 22 knots you need an additional 50 hp to get over the hump, then you can throttle back to approx. 80% power.

And your boat should go on a diet, with 3.8 tonnes it is overweight.

apology's cdk, i had a prop manufacturer run my figures through there computer today and that came up with a top speed of 10 knots, so your conclusions aren't far off there's. i only wish i knew as much as i think i do. the 2h diesels i was going to use would have been ideal but when i measured the boat they were to high. if i can cruise at 8 or 9 knots i will be happy so i will go ahead with the 4 cylinders. another interesting thing they told me was my boat weighs 4.5 metric ton.

[CDK]

Originally Posted by whitepointer23

another interesting thing they told me was my boat weighs 4.5 metric ton.

Quite unusual for this hull length. Is there lead somewhere, depleted uranium or concrete spiced with bolts and nuts?

[whitepointer23]

yes it does sound heavy, these boats were built in the 70's when more glass meant more strength. probably the reason they are still sought after, they last for ever, you just keep repowering them. if you add up 2 v8's, 400 lt petrol. 250 water. hot water heater, there's 1.5 ton. so the boat must 2.5-3 ton on its own. i would have thought 1.5 for the boat on its own. it is the second time i have been told it weighs 4.5.