A question for power boat designers

[Gordon Scrim]

Hi All,
I am interested in building the Riviera that is on the svensons site ( http://www.svensons.com/boat/?p=RunaboutsInboard/riviera) but would like to move the motor backwards. I would like to flip the motor around and drive the boat with a V drive. This would enable me to make a larger cockpit and use up some of the wasted space in the rear whilst still using the same shaft angle.

My concern is that moving the motor backwards about two feet would upset the boat in some way. I am not a boat designer but cannot find the boat I want so I am going to have to modify an existing design slightly.

I have also looked at the Tahoe 19 from Ken Hankinson and I asked him about moving the motor to the rear but he was against it. I suppose there are degrees of moving and some would not upset the boat but I could understand that moving the motor to the transom would indeed upset the balance and buoyancy of the vessel.

Any ideas/comments?

regards
Gordon

[mmd]

Hello, Gordon;

As with other boats, good performance in a speedboat is dependant on balance. In the case of what you are wanting to do, balance has two forms that you will need to pay attention to. First will be static balance - if you shift a major weight aft without adjusting the volume of the hull aft to compensate, the boat will float down by the stern. This will affect the exhaust tip placement, bottom paint and boottop location, and swim platform location (if fitted). It will also make the boat awkward to handle at displacement speed, as her nose will be high in the air and her aft end squatting a bit, resulting in poor visibility ovr the bow and sluggish turning.

More important is the dynamic balance of the boat while underway. As the hull moves through the water, pressure builds up on the bottom and lifts the hull out of the water. This dynamic lift acts mostly on a rather short section of the hull just aft of where the forward portion of the hull makes contact with the water surface while at speed. The boat balances on this area like a plank laid crossways on a sawhorse. The remaining portion of the immersed hull provides little lift; mostly directional stability and a damping effect on pitching. Everything in the boat must be placed so that the boat balances more or less just aft of this lifting area of the hull, or there is a performance penalty to pay.

If the centre of gravity (CG) of the boat is too far forward the boat will have a hard time lifting its bow up over the bow wave and getting on plane. When it does get on plane and gets driven to the speed at which the hull form is most comfortable, the area of lift is behind the CG and the bow drops and the boat slows down from the added friction. The centre of lift moves forward until under the CG, the bow lifts, and the centre of lift moves aft until enough lift is generated to bring it back to the "sweet spot", where it is again unbalanced and the bow drops. This is called porpoising and is most uncomfortable.

If the CG is too far aft (which may be the case in the changes you are contemplating), the bow will be carried too high, causing the angle that the planing hull approaches the water surface to be too steep, resulting in slow speed and a lot of splash and fuss (not to mention poor visibility, bad gas mileage, and miserable steering). It can also cause the hull to be "pooped" easily - this happens when you slow down and come off plane suddenly and the wave that you are pulling along behind you catches up and washes up over the transom. This can be nasty, sometimes.

Boats that are designed for the motors to be placed well aft have broad transoms and fine entries; From the brief glance I took at the pictures on the website you linked to, it seems that the Riviera has a rather bluff entry and a somewhat narrow stern. Hers is a shape ideal for the weight to be centred near midships, not aft.

All that depressing comment being said, bear in mind that my comments are, at
this stage, just a hunch based on past expeience. To be sure, I'd have to have a peek at her linesplan.

[carioca1232001]

mmd wrote:


First will be static balance - if you shift a major weight aft without adjusting the volume of the hull aft to compensate, the boat will float down by the stern...

More important is the dynamic balance of the boat while underway. As the hull moves through the water, pressure builds up on the bottom and lifts the hull out of the water. This dynamic lift acts mostly on a rather short section of the hull just aft of where the forward portion of the hull makes contact with the water surface while at speed. The boat balances on this area like a plank laid crossways on a sawhorse. The remaining portion of the immersed hull provides little lift; mostly directional stability and a damping effect on pitching. Everything in the boat must be placed so that the boat balances more or less just aft of this lifting area of the hull, or there is a performance penalty to pay.

If the centre of gravity (CG) of the boat is too far forward the boat will have a hard time lifting its bow up over the bow wave and getting on plane. ....

If the CG is too far aft (which may be the case in the changes you are contemplating), the bow will be carried too high, causing the angle that the planing hull approaches the water surface to be too steep, resulting in slow speed and a lot of splash and fuss (not to mention poor visibility, bad gas mileage, and miserable steering)........

Very interesting subject. I have the following questions:

1. Is there a single book you could recommend - and that is readable by non-marine-architects - where the elements of power boat design are presented ?

2. Is it possible to make a working estimate of the location of the C of G by hoisting your boat on a travel lift and reading off the weights as registered on the fore and aft slings, plus the spacing between the latter ?

3. Imagine a 32 ft semi-displacement wooden motor-cruiser, equipped with 2 X 170 HP/3600 rpm petrol engines, coupled to 1.5:1 gear boxes; if you were to go for 2 X 150 HP/2600rpm diesels,what gear box ratio would be optimum, assuming that old props could be discarded and new ones installed if need be, so as to retain original performance ?

[Gordon Scrim]

Hi mmd,

Thanks for you information about the problems that could be encountered. You description is very clear and even I understand it!

One of the points that I did not like too much about this boat was the "bluff" (well put) entry - but the lines of the stern are quite nice.

I suppose if this design was to be used as I suggest I would have to modify the lines of the stern. I think that would require a navel architect type of person to modify the lines correctly so I did not end up with a dog.

regards
Gordon

[mmd]

Carioca:

1.) My favourite is a small paperback by Frank Bailey titled "Small Boat Design for Beginners", published by A. Reed Books in 1980, but I believe it is out of print now; a quick Google search didn't turn up any hits. Lindsay Lord's "Naval Architecture of Planing Hulls" is a seminal work on the subject, but I see that copies are now fetching US$200 and up. There is a book by Roger Marshall titled "All About Powerboats" (available here (http://www.marineengine.com/books/b2.html), among other places) that seems to cover all the right topics. I have not read it, but the index shown on the website I linked to shows a comprehensive subject range.

2.) Yes.
http://www.imagestation.com/picture/sraid158/p5095bbbd43133ffce43abdf406caea7c/f51e6c89.jpg

3.) There would be a performance loss because the new motors have 12% less power. As for prop sizes and gearbox ratios, to run the calculation I would need the following data:

LWL
BWL
Draft (at rest)
Displacement
difference in weight between old & new motors
.
.
.
Gordon:

Taking an older, mid-engined hull form and tweaking the lines to add buoyancy aft is not a big job for a competent designer, as long as you don't want him/her to re-draw the whole set of drawings to suit the new lines. I would expect that it would cost you a few hundred (US) dollars, which is small in comparison to the value of the boat.

[carioca1232001]

Thanks mmd for being so helpful.

Please specify what LWL and BWL mean - I am basically a land-lubber and new to boats. ;)

I shall then forward all the data you require for the calculation.

Thanks again

[Bruce Hooke]

LWL stands for Load Water Line and means the straight-line distance from where the hull cuts the water's surface at the bow to where the hull cuts the water's surface at the stern, measured with the boat at rest with a normal load on board. The LWL should be specified on the plans. Here's a nice little picture I found on the web (ignore the stuff about incline angle -- that has to do with installing a product being discussed on the web page (http://www.oceandata.com/support/install/install-tank.htm) on which I found this picture):

http://www.oceandata.com/support/install/images/lwl.gif

BWL is the same thing measure across the boat (prependicular to the LWL) at the widest point.

[carioca1232001]

Thanks for the LWL and BWL explanation.

At the present time I am re-assembling all the equipment that is to be re-installed in the bilge, which means engines, gear-boxes, hydraulic steering, rudders, fuel tanks etc etc.

My boat has been on stands since early last September when I started to re-fasten/re-plank the undersides and my plans of getting it ready for summer went under as we had an unusually rainy summer this year. This wrecked my schedule for painting the insides of the bilge area, but at the same time also afforded me a much deserved rest.

Once I get mky act together, which should be no later than 3 weeks from now, I shall come back and reactivate this thread.