Glastron gs249

Hi

yes deep v's do need more hp to produce specific speeds but needing more hp to achieve certain speeds isnt always due to increased prop slip, if you view drag from the deep v hull as merely more weight it will take a more powerful engine to achieve specific speeds.

In efficient racing hulls they are all not deep v's ( lake racing etc) as they are looking more for top end speed and less drag than the need for offshore stability.

Racing hulls also employ things such as stepped hulls which produce air pockets under the hull etc to reduce drag to gain speed and with less drag less prop slip. Also racing hulls are designed with weight a big issue and again lighter hulls cause less drag and thus aid in increasing speed and reducing prop slip.

racing hulls are also designed purely for performance efficiency, not having to worry about comfort, ride etc etc so they are less compromised than pleasure craft and more targeted for their specific need making them more efficient and hence the lower prop slip.


Thats my view but what do I know!!!!

Kevin

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Re: Lulworth ranges.....

Lottsa fun. Head out of Poole area towards Lulworth Cove.. When large boat appears, pretend you don't have radio. Especially when they hold up big card saying Channel 13. They'll then turn on 50000W audio system, and play nice music at you until they find correct button to push to tell you to go at 240 deg for 5 miles.

Play game and go 240 for at least 2.5 miles out past second patrol boat. Wave at nice people in trawler patrol boat 2.5 miles out. Go on another 0.5 miles.

Then, making sure you have at least 3 boats in company capable of greater than 40knts, open up and do a Red Arrrows type 'peel off''

Patrol boats will slowly open up to maximum speed of 14knts.

After arriving in Lulworth Cove for lunch, await arrival of Patrol boats, and deny all knowledge of reckless idiots driving through range, and listen to whizzy sound of shells going overhead.

Have never done this exactly, but I'm sure it would be fun, and irritate them enought to sink you on the way home /forums/images/icons/laugh.gif

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My hull is not racing, but has steps, and a bunch of clever stuff in between, but also an extremely deep V forward. Result is a boat that rides high at speed, but has wave piercing abilities of deep V, and good displacement speed abilities..

However beam is wide for a boat of this length, with excellentcockpit room. Good all round compromise, and came out technically as best hull in a comparative exercise in one of the motor boat mags last month, though lost out on overall winner as expensive compared to hulls that didn't perform as well.

As someone that's pushed it, it does everything that it says on the tin, and sometimes beyond.

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Just been thinking about what you saying about 330hp /300 torque car would be quicker than 200hp /400 car, now Ive had a few beers but looking at this in reality your theory you would have to be talking in perfect conditions.

For example japenese hot hatch 220hp at 7000 revs hitting 120mph torque about say 130 or something (precise figures not important its the theory im concerning myself with)- fine on the level but stick 4 people in it and say go uphill hasnt got the torque to be in 5th gear and still hit the 7000 revs needed to hit maximum bhp and thus cannot hit maximum speed, drop down say two gears to allow the torque to bring the revs back to 7000, but immediatly the theoretical top speed has dropped because of the gear ratio difference between that of third gear as opposed to fifth.

Now say a car with 160hp with 220 torque reaching say only 105mph on the flat- load it with 4 people and go uphill, the torque can possibly overcome the extra load put on it and thus the car can stay in fifth and stay within its maximum power range givng it a less drop in speed from its 105mph maximum. thus possibly the lower bhp but higher torque car will out perform its higher bhp rival.

Therefore in certain conditions lower bhp with higher torque will out perform higher bhp with lower torque and I believe that works the same for boats.

if you have high bhp and low torque in a heavy boat you may have to do the same as you would in a car and drop ratios which in a boat would be a drop in prop pitch which again drops the maximum theoretical speed it can possibly achieve ( max operating engine revs devided by gear ratio multiplied by pitch of prop express that in yards devide by 1760 multiply by 60 gives you it in mph, is the simple calculation I use to work out maximum theortical speed of a boat)

A boat with lower bhp and higher torque could well have enough torque to throw over a larger pitch prop giving it possibly a higher theortical speed

Surely this is why, not in terms of acceleration, but top end speed, diesel boats ultimately can get near their petrol engined rivals without having neccessarily to produce the same outright BHP, they use the torque to produce the performance


Kevin

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Without knowing your boat my guess is its a regal, not many stepped hulls over here.

stepped hulls do give a higher top speed but if done badly and purely for top end gain can be dangerous and unstable, as has been proved in the USA with some of their monster speed machines. but from what you saying regal have it spot on!

Kevin

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Yes, it's a Regal, though that's easy to work out from my profile. There are many stepped hull design's that work, though maybe not many as well as the fastrac?

They are not a racing hull, and perhaps that's why they work. Not designed as a go faster dangerous hull, but as a good all rounder

I, and others who have offered to jump ship to a more comfortable boat, with many years of experience of boating than I have, think it rides well in many conditions

No one will ever agree, it's a subjective thing, though I'm now spoilt, and would see a Windy as one of the few upgrade paths if I stay a stinkie.

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...exactly.

Top speed is a function of power, but that isn't helpful if you can never get planing so that the engine can run at the rpms required to produce max power. But, how often do most boat owners run flat out, anyway ?

Power = torque * speed (sort of)

Max power is measured at full throttle, at one engine rpms.

What it doesn't tell you is how much power (or torque) is generated at any other rpms, which is what you are very interested in when trying to answer the question "Will my boat get planing with 6 people and 4 cases of Wolf Blass on board". For that, you need to know the power (or torque) generated at whatever revs the engine is running at the "hump" speed before planing.

Take look at the power & torque curves, and it all becomes a lot clearer. For their size, diesels often produce a goodly spread of torque lower down the rev range where you need it, whereas petrols tend to produce more torque higher up the rev range (you can minimise the effect by de-tuning, which is partially why you have 5-litre petrol engines producing only 220bhp).

"Gearing down" can help make peaky engines more manageable, but all you are doing is squashing the curve - it will still have a "bump" in its power delivery at the same relative part of its rev range.

Petrols are fine for those with deep pockets, high performance applications where you are massively overpowered anyway, or on small stuff.

dave.


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I agree with what you say.

In a way you have summed up what I was trying to explain, bhp on its own is fine on easy plane boats or light boats but for performance in heavy or ladened boats torque, in my opinion is of more importance.

example being say 220 hp outboard 2 stroke with specific pitch prop ( high power virtually no torque) fine on sports boat /light boat. but take a 24 foot cruiser (weighin in at say 2500kg) with say a 4.3 v6 220hp with same pitch prop - the same bhp as the outboard. with its 220hp inboard it will say hit 40mph and accelerate fine as it has the torque to reach optimum revs thus releasing its full bhp and top speed, take out the inboard and stick the outboard on the back it will struggle and i doubt would ever reach its optimum revs to produce its optimum bhp and its maximum theoritcal speed as it simply wouldnt have the torque to move the boat forward quick enough to reach the required optimum revs and thus produce the 220hp. Would have to change the ratio which would mean going down in pitch of prop to allow outboard to rev higher to hits its optimum bhp.



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I agree with what you say.

In a way you have summed up what I was trying to explain, bhp on its own is fine on easy plane boats or light boats but for performance in heavy or ladened boats torque, in my opinion is of more importance.

example being say 220 hp outboard 2 stroke with specific pitch prop ( high power virtually no torque) fine on sports boat /light boat. but take a 24 foot cruiser (weighin in at say 2500kg) with say a 4.3 v6 220hp with same pitch prop - the same bhp as the outboard. with its 220hp inboard it will say hit 40mph and accelerate fine as it has the torque to reach optimum revs thus releasing its full bhp and top speed, take out the inboard and stick the outboard on the back it will struggle and i doubt would ever reach its optimum revs to produce its optimum bhp and its maximum theoritcal speed as it simply wouldnt have the torque to move the boat forward quick enough to reach the required optimum revs and thus produce the 220hp. Would have to change the ratio which would mean going down in pitch of prop to allow outboard to rev higher to hits its optimum bhp.



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yes many stepped hulls do work problem is that in the states, and im talking about 27-50 foot 1000hp+ 120mph monsters ( Sunsation, Jaguar, Fountain, Cigarette, Nortech, outerlimits, Pantera, American offshore, eliminator etc etc) many manufacturers simply jumped on the bandwagon and produced stepped hulls without looking at the designs behind them all in the name of simply getting more top speed to be able to promote their boats with but unfortunatly produced some which some people have claimed dangerous with no stability, all in the name of top end speed, the better manufacturers looked at the benefits of stepped hulls and designed the hull to encompass the stepped design but without compromising safety, handling ride etc.

In th States in offshore performance circles the tunnel design is the latest development and seems to be the 'new' stepped hull.





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