Help With Economical Semi-Planing Designs

Looks like maybe 10 tons at 31 feet and 20 knots at one gal per hour. I'm impressed

Actually, I'm more like a caveman stepping out on the freeway at rush hour. What have all the other designers been doing?

 

Ron --rule of thumb but pretty accurate


diesel 5galls per 100hp used
petrol 7 gall'' '' '' '' '''
2 stroke 10galls''' ''' ''' ''


It doesnt matter if the cat is in a generator or a truck at "full" 320HP it has to consume 16galls.

The trick here is HP 'used' not the size of the engine.

 

My 2001'st post and I hate for it to be a negative one, but sorry Longliner, I'll take Tom's polite scepticism and raise him.... Unless this jc hull has rewritten history, then that just ain't possible. The only boat I can imagine getting 20nmpg is a sailboat with it's genset running to boil a cup of tea..... in which case it's more likely doing 2 knots, not 20.
I think you said the boat you were aboard had a pair of volvo's with dp legs. If we assume that they were the 6 cyl 200hp engines, then even if 20 knots only required 1/2 of their max output, combined they would be consuming about 10gph. Which gives us a more realistic 2nmpg. In order to provide the sort of economy you're suggesting, they would only be producing about 5hp each!

 

Okie dokie - here's a repower from 3208 T Caterpillar Diesel with 320 horsepower to a 350 HP 6LYA-STE Yanmar.

"I was delighted at the final results. Kristen went from a 15 knot cruising speed at 12 GPH to 22 knot cruise at 14 GPH. The top speed went from 20 knots to 25 knots."

So with the 3208 Cat he cruised at 15 knots @ 12GPH. So that's 1 1/4 NMPG.

Doesn't look like you're too far wrong Jack.

Best,

Leo

 

JC boat sales,,,,10 progress ave Nashua new hampshire USA,,tel,,603- 886- 0300,,my 32 ft sailboat weighs 10500lbs and its made of wood,,,longliner

 

found this too ,,might have been a little hard ,,they moved part of thier operation to fla ,here goes,,,,,,,powerandmotoryacht,com look under 31 jc ( boat test)also says at 6 knots 1gph,,and at 20 knots 8gph,,but that is with conventional single screw drive,,,,not duoprops with twin turbo 4 cyl volvos,have fun ,longliner

 

Hey ,,,,,were did ya go?????????dont ya just hate it when im right,,,,longliner

 

:?: :?:

Ok - lets put this JC31 thing to bed. I'm not sure where you're getting your numbers from Longliner, but here's the specs on the JC31 as published in Power and Motoryacht

They clearly show that this boat is no more economical than any other of the type. Indeed, whilst it might be a fabulous boat, I'd consider it to be pretty bloody thirsty! At the twenty knots you refer to, powered by a single 370hp shaft drive diesel, the engine is consuming 19.5 usgph - equating to 1.03 nmpg. Even allowing for the greater efficiency of the dp sterndives (which would be offset somewhat by having twins), there's absolutely NO WAY that it could use almost 20 times less fuel at the same speed........

 

Every one has told you your fuel consumption figure are unrealistic. explaining with formula.

You refuse this-- Whats left to say?

 

yes will you can ,,with the twin model210 turbo charged diesles ,,and the counter rotating duoprops ,,with 0 cavitation buy the way ,,it can be done .and was done,,remember none of you belived the 31 ft boat could weigh 11000 lb,, one said .it was more like 11 tons,,we also ran the engines at 3300 rpm.our engines were pretty light also ,,2 men could pick one up ,,im not going to defend myself any further ,,I know what I did in my life ,,on these boats,,if you refuse to believe ,no problem ,,if you want to learn more ,you have JCs corporate address,,,and please tell me how I could fish 100 miles offshore for 9 or 10 days,,,with 375 gallons of fuel,,, and jack you say 5 gallons per 100 hp.jc is showing on the above chart 1 gph with 370 hp engine 6 knots ,,not shabby ?longliner

 

Look - I'm sorry - I'm not trying to have a go at you.
Ok - so these were 4 cyl, 210hp a piece, running at 3300rpm. According to the Volvo website, they never made such an engine: their largest 4 cyl being the fairly recent 170hp KAD32P.
But if we look at one of their current engines, the 225hp D4, at 3300 rpm the engine is producing 180hp and consuming 10 usgph. 2 such engines would of course be using 20 gph, so your JC31, if it was doing 20 knots would be getting 1 nmpg. Now it's possible - though I would contend somewhat unlikely - that Volvo have regressed somewhat and their new engines use 20 times as much fuel as their old ones.....

 

5 gall per 100HP used not the size of engine.

It is quite likely your 2 Volvo 210Hp engines would use almost 1 gall per hour just standing stll in neutral sat in the harbour.

 

Yes - I use 20hp per gallon per hour - so the same thing. It is of course only a ballpark figure. The D4's that I quoted, which are one of their latest and greatest, are only managing 180 hp/gal/hr (taken from their prop curve)

 

Hey Longliner-- I think these guys are missunderstanding what you are saying. I take it that you are saying that if you where out fishing for 8 days or 200 hours, you would average about 200 gallons of fuel useage.
Or about a gal a hour.

They think you are saying that you could run 20 knots a hour and use one gal a hour to do it. Thats impossible.

This duo prop counter rotating props with 0 cavitation is interesting.
As I recall a outboard has around 20 to 40 % slip.

 

willalisson anjack and all others ,,,I was wrong ,we got a gallon per hr with a 6000 lb payload ,,,,,but the controversy that stuck in my mind was that at the time fishing boats only went 6 or 8 knots,and burn allot of fuel ,,,but our boats could go 20 knots 26 at top,,,,,,,,,,I was wrong you were right ,,I am a firm beliver of truth,we were also told by volvo that no matter what the boats could only do 40 knots because of phyics,,that was proven wrong by someone one this site that set a speed record ,,with one duoprop ,,the picture shows the boat flying with only the prop in the water,,,,,,Ill get you all back later,,,,,lol.....longliner,,,,,dont ya just hate it when jack is right?and yes ron the tecknology is from ww2 topedoes,so they would go staight, and behind the boat that is what it looked like...