Stretching of a hull / relocating the CG

Remnant brought up in another post the issue of lengthing a boat design he wanted to build. And I've seen articals regarding larger yachts that have had sections or cockpits added. How does one know how this will affect the performance and/or ride.

The hull Remnant mentioned was the Grand Slam 38, apparently at one time called the Daytona or ??? I would like that design if the hull was 6' longer with all of the added length in the form of a longer cockpit.

Another idea I've contemplated for a while was to find an older model offshore boat and reconfigure it to more resemble an express cruiser. I really like the way the Grand Slam helm is configured so as to be surrounded by a raised coaming. It reminds me of how a PT boat looked and that's the look I'm going for.

I thought that I could get' for example' a 40 foot early 80's Formula offshore hull (relatively light and inexpensive), and redo the deck and cockpit liner and add a single diesel using the FEYS electric system connected to two Arneson surface drives. The single diesel would be mounted forward, under the sole of the new raised helm.

How would such a major shift in weight affect the performance? I'm looking for a 30-35 knot cruise, top speed isn't important, fuel economy and minimum wake are.

My boating hours are spent all inland, San Francisco bay and the California delta. "Big" water is not an issue for me.

You can't hurt my feelings, so thanks in advance for your candid responses.

Dennis

 

Deep vee hulls need horsepower, lots of it. If you add flotation aft and weight forward it won't even get up on a plane. Also, if you want little wake, this is not the boat for you. This type of design with equal deadrise along the bottom makes a huge wake. Also, electrical drives with the generator are very heavy for the power. They are great on tugboats or trawlers, but a Formula needs power with light weight.

 

Shifting LCG on Planning Hulls

When designing planning hulls one of the designers primary functions is to make the best possible compromise between conflicting requirements. It is important to Identify the intended use, and expectations for the design in question, then some criticle decisions can be made.
Many perameters effect a boats planning performance such as Deadrise angle, buttock shape, Mean beam between chines, wetted chine length,LCG, Lcp,CLR,Displacement,Beam at transom,section shape, wetted surface, etc... Just looking a one or two perameters when contemplating a major change to a hull such as adding wight and shifting tha LCG will have a major impact on performance.
In general adding weight to a planing hull is never a good thing it will just add to the power demand to reach the design speed, which will increase the need for more fuel to maintain the desired range which will add more weight and so on it goes.
Shifting the possition of the LCG without a study of where the boat will trim in equallibrium thru out the performance envelope is like playing Russian Roulette. If the LCG is too far forward Trim angle will be reduced from optimum and Bottom pressures will be lower and greater resistance will result, as Gonzo said the boat will not even get on to a plane. If the LCG is too far aft then bottom pressures will be higher and the boats trim angle will be increased from optimum and could result in dynamic instability of an ocillitory nature, i.e. the bow will want to continue to trim up until a bow down moment is developed from the center of pressure moving too far aft, and on and on it will go.
When making changes to planning hulls try to think of the whole enchilada so that unexpected characteristics do not develop.

 

Gonzo,
According to Fast Electric Yacht Systems, their system weighs less then a standard dual engine installation of the same HP AND torque. They don't list weights so I can't be sure.

I think I understand that it's going to take HP to get a deep vee hull on plane and keep it there at slow speeds. Maybe the sample hull I listed is too deep of a vee, (24 degree at the transom).

Doug,
It sounds to me from your response that I might be better off starting with a hull that was designed for the approx. speed/engine placement, etc., as I'm looking for. Problem is I just can't find that boat. And I don't believe after surfing this site for the past few weeks and the homework I've done elsewhere that I could afford to have one designed and built.

So everyone,
If I'm looking for a hull, approx. 35-40' LOA, fairly narrow beam , that will plane quickly with a minimum of wake (Doug you must be familiar with the delta), cruise economically at 30-35 with I hope can be a single 480 HP diesel with dual surface drives, what should the hull spec's be, i.e., beam, deadrise, weight, etc.

Dennis

 

I'm not familiar with Fast Electric Yacht Systems. If they are lighter it would interest me to get more information. Is the drive system alone lighter or the whole power package?

 

Dennis,

A hull form that may fit your requirements might be found at
mangiaonda.com
They describe a "M" hull shape that recaptures the energy of the bow wave, and converts it into lift, and in doing so midigates the divergent wave train. They claim planning speeds for their 40 footer to range from 10 to 35 knots with an aeroplanning mode in excess of 40 kts.

Doug

 

10 knots is too slow for a 40' boat to plane. Assuming the waterline lenght is 33', the hull speed is about 7.2 knots, times three is 23.6 knots-the minimum speed considered planing.

 

Doug,

I looked at the Mangiaonda website. Not the sexiest boats, are they. But I can see how they would plane at lower speeds. Not a vee hull at all.

Gonzo,

Could shortening the waterline length help and to what degree?
Could a transom be notched some to reduce the waterline length while underway, but not at rest? The Arnesons could also be set forward. You mentioned 33' waterline length = hull speed of 7.2. How did you arrive at this? Is the formula linear?

Would the leverage from having the prop as far back as it is with an Arneson drive help push the stern up and the bow down to plane faster? Would it be able to keep the bow lifted once on plane?

How much wood could a wood chuck chu...........?

You can just guess at the last question.

Dennis

 

I you design a boat with a shorter waterline for the same displacement, it will start planing at a lower speed. However, the bottom loading will be higher consequently the horsepower requirements will be higher too. This will require bigger enginesm which are heavier, which require more horsepower....
There is a range between which all this works.

 

Sounds to me like you're after a hullform more reminiscent of the Huckins Fairform Flyer ( www.huckinsyacht.com ) - essentially a warped plane, shallow vee. Besides, they built real PT boats by the hundred......

Gonzo - more info on the FEYS diesel electric sysytem at http://www.feys.org

There was also quite a detailed explanation of a FEYS installation in a 42ft Ocean Alexander in one of the more recent issues of Passagemaker magazine. The system is essentially derived from those used in trucks and busses. The biggest problem as I see it is that each unit can only absorb 133hp, though they can be 'grouped'. So for a planing boat of the size that Dennis wants, you're going to need quite a few of them.....

 

Willallison,
I figure I would need 4 motors connected to 2 outdrives (2 motors per drive) to be driven from a single diesel. The engine would be approx. 600 HP.
I've seen the Huckins before and from the sheer line down it is pretty much exactly what I envision except a I would like a lot less freeboard as mine would be an express cruiser. And I would want less beam.
Mine would be a day boat, no head, no galley, a simple vee berth because I wouldn't know what else to put there, minimal fuel as I would never be more then 3-4 mile from a marina and I couldn't burn a large quantity of fuel in the delta in a day if I tried.
Dennis

 

Rule No.1 of succesful boating Dennis:

If you want your wife to go boating with you, NEVER buy (or build) a boat without a comfortable, private loo !:!:

 

Willallison,
We have a boat now that doesn't have a head and my wife wears this like a badge of honor. She brags to friends that have a head on board about the fact that she doesn't have to clean or smell a head.
When we boat, it's warm. If it's cold, we're home. If she (or anyone) has to use the loo, it's like two minutes to the next marina.
I doubt she would let me install a head if I begged!
And I feel that to get the performance I want out of the size boat I want, weight is going to be critical. No head= no water tank, no gray water or tank, no plumbing.

Dennis

 

Watch out Dennis, you'll have a lot of old salts wooing your wife.

 

I'm reviving this topic because the design may have mutated to the point that this idea may now be semi-feasible. Your comments are welcome.
I have another topic going about the design of a 40' power boat and my original idea was to have the engines forward as described above. That design now shows stearn drives so the weight shift may be gone.
If I were to start with an early hull as I describe above, leave the engines where they are but just reconfigure the deck and cockpit to a more "cab forward" or "cockpit forward layout, how much would this affect the LCG?
I feel I would be losing weight from the hull by losing much of the interior, but would be adding passenger weight a little more forward. I would think that the dry weight of the hull would be down.
I would like the end result to sort of resemble a PT boat. Doug Frolich did some renderings that he has posted in the gallery that are like 99.9% dead on.
Could I not purchase an older hull, remove the entire deck and have a new one made, or cut out some of the original deck and graft on a new section with a dodger of some sort?
As I can't afford to build a new boat from scratch, this may be an alternative I could afford, but I don't want a boat that will perform badly or worse yet be dangerous.
Dennis