Complete Beginner's Questions

[PAR]

James, there are several things to consider when developing a powerboat hull form. Target speed, general sea conditions expected, weight, construction method, etc. Define you requirements then find a suitable hull that's intended for this. The racing hull forms can lead to fast boats, but also uncomfortable rides and huge power requirements. Maybe styling clues are better, then actual hull forms. This way you can have well tested form,with race boat styling, but will not knock your dental fillings out in a moderate chop.

A moderate constant deadrise hull form of say 18 degrees or less or a classic warped bottom if target speeds will be lower then 35 knots.

[James88]

Olav helped me a lot with all the technical knwoledge and gave me lots of advice. Thanks again! I combined the technical advice and my own design ideas into some rough hull sketches. (They are still rather some stlying examples.) I also created some basic sideview line drawing. The hull is now based on a typical V-Bottom powerboat hull. Comments and crits are welcome!

[marshmat]

Definitely an eye-catcher. The bow profile, the transom slope, the colour scheme, it just screams "money and class".

While I wouldn't go so far as to specify a particular drivetrain at this stage, your mention of the Merc HP1075 SCi and the UJ410 jet is interesting. This would be a very expensive drivetrain, but one that would tend to get you going in quite a hurry. A supercharged racing V8 coupled to a large-diameter jet would likely provide absolutely insane acceleration.

It will also provide insane deceleration. So I'd give some serious thought to where the wake is going to go when you stop. A slick reverse transom looks sexy, but if you jam on the brakes (and with jets, you really can jam on the brakes- full reverse at full throttle) then all that water behind the hull will come right up onto the swim platform, possibly farther. This happens at the same time as the bow rises and the stern squats- potentially dangerous if the transom shape allows that water to come aboard, especially in a relatively stern-heavy speedboat.

I've never seen a jet drive double-step hull, and can think of two problems that this combination could introduce. The first is that steps introduce air- that's part of what makes them so effective. But jets don't like air- if it gets sucked into the intakes, the impellers start to lose their bite on the water, rpm goes up, and thrust goes way down. The other potential problem has to do with directional stability. With a rudder or sterndrive leg in the water, a prop-driven speedboat has some lateral resistance to help it track properly. With jets, you don't have that lateral area, and it might be necessary to compensate with additional appendages.

With 2150 hp in the engine room (that sounds like more than a bit much for a 38-footer, to be honest I think half that might be more typical), you're looking at something like half a square metre of engine compartment vents. It will be an interesting challenge to figure out how to get that much vent area integrated into this kind of sleek, low styling, while preventing them from acting as water intakes.

I'd also add that the starboard bow light should be green and the port bow light should be red.... but at this stage that's just nitpicking

[James88]

Thanks for the input.
I'll think about the problems mentioned and try to solve them. There still quite a lot of work to do anyway. I'll keep you updated!

[Olav]

Sorry, I was not at home last weekend so I wasn't able to answer your latest e-mail. Instead, I'll now comment here.

Marshmat is right with his concerns about the steps in combination with waterjet propulsion. Apart from the potential disturbance of the flow towards the jet intakes, steps are very tricky to locate; there are several threads in this forum about this topic and afaik none of them came up with a "real" answer to that problem. That's why I would abandon the idea of putting steps to the hull - at least if waterjets are a "must have".

I also see several problems with your propulsion plant. First, the UJ410 is not capable to absorb the power of the Mercury (the maximum input power is 671 kW, the HP 1075's max. output is 801 kW @ 6000/min). You'll also need big transmission gears to reduce the WOT-rpm to a value usable for the jet (the UJ's performance chart ends at 2200/min) which are additional weight and cost, waste of space and power. Furthermore, you might get into trouble with your longitudinal centre of gravity which you may not be able to get aft enough if the engines are too far forward due to the extra distance in the drive train from the transmissions.

From what I read about the Mercury HP 1075, it rather seems to be an engine for pure racing application, so it may not really be suitable for the average boater with only little knowledge about how to handle and treat such a machine. The manufacturer's limited warranty of only 90 days concedes the case for this... In addition I wouldn't install anything other than a diesel as an inboard engine because of the risk of petrol vapour accumulating in the bilge, providing a potentially explosive mixture with oxygen from the air. It wouldn't be the first boat to be blown up accidentally by just a little spark...

Maybe you could tell us your performance requirements and main particulars of your design to make it easier to suggest a suitable propulsion plant.

[mydauphin]

Another problem is providing enough fuel to get anywhere with it.
I would do transom at 45 degrees so reverse thrust lifts boat. I did a 27 foot i/o drive boat similar to yours and owner almost sank boat within 5 minutes of taking delivery by reversing at full throttle at boat ramp.
An idea I had for step hull was to split them like catamaran and leaving center with no step but you still might have problem getting enough water.
I would also extend platform over jet, so you could sunbath or fish or do something back there.

[James88]

Hi Olav... no worries.

mydauphin: True... well I'm not yet sure about the transom anyway.
Is there any option to limit the reverse thrust? (Does this even make sense?)

The actual design goal of the boat is to create a Powerboat/Luxury Yacht crossover. Therefore my idea was to use the engines of a real powerboat. However your doubts seem to be really reasonable.

My main concern is to create speeds and driving characteristics similar to a power boat. I'm aware that this boat is most probably going to be heavier (due to the onboard luxury applications...) and will probably be not as fast as a powerboat after all since aesthetics are more important than pure technical optimization.
I'm not sure what speed a 40' powerboat with 2 of the proposed Mercury engines usually reaches. My thought was to get my yacht up to a speed of around 60-80 knots.

[James88]

I'm currently doing some hand sketches to explore some shapes and features.
Just a quick update thoug. I'm doing this project in addition to my usual uni work, so everything is going quite slow as there's not much time to spare.