How to stretch the hull 21-OCT-2013
The following Glen-L articles discuss lengthening a boat and changing plans :
Lengthening a boat (WebLetter 38)
Changing Stock Plans (Ken Hankinson)
Here are a few important points loosely quoted from these articles :
- The frame spacing is increased but the stem is left as per plans.
- From the plans, determine stem length, frame spacing and hull CB. The information is usually on
the profile page. The drawings are to scale, usually 1"=1', so the measurements can be taken directly from the plans.
- The correct way is to re-space each of the stations or frames from the aft end of the stem to the transom a
proportionate amount up to 10%. If frame spacing is equal or varies, multiply by the percentage factor being used. As an example
if frames are uniformly spaced 18" apart and the 10% factor is used, each spacing will be increased 1.8" (.10 x 18") or rounded
off 1 3/4" (1.75"). ]
- Up to the 10% factor, the integrity of the design remains basically unchanged.
- All length factors are altered 10%. This means the location of a centrally located engine/transmission CG
(center of gravity) and location of major weights are also shifted by the 10% factor.
Lenghtening a hull is discussed on Glen-L's Boatbuilder Forum in Neel Thompson's Gentry Build thread :
Gentry build continued
How to stretch the hull past its design length
Ever want to stretch a boat design past its design length ? We have a need for another boat to break in and test
our custom marine engines. After surveying the used fast boat market, I came to the conclusion that we'd be better
off building another boat. We've been considering stretching the Hot Rod from 16'-6" to 21'. At the higher
speeds we've been running (80 MPH +), extra length would be welcomed. Glen-L does nor recommend extending or shortening
the Hot Rod's hull nor increasing the beam. If we choose to stretch the hull, we'd be totally on our own on this
one. We would have to find the stretched hull's CB, rebalance the weights and hope that the new boat will handle as
well as the original.
Flotation Regulations & Hull Length
In Canada, boats under 6 meters (20 feet) in length are required by law to have sufficient flotation foam to float the boat if
swamped. Boats longer than 6 meters are exempted from this requirement. It can be quite a challenge to fit
8 cubic feet of flotation foam in a 17 foot boat. Stretching the hull to 20 feet would circumvent the need for flotation foam.
Hull Stretching Methodology
We created an ASCII file from the Hot Rod's frame offsets and imported the offset file into HullForm Version 9. Not the
latest or greatest software, but it's free, easy to learn, well documented and produces hull hydrostatics. The hull was
stretched from 16'-6" to 21'by multiplying the frame spacing (21") by 110%. The rendered drawings included below are very crude
but demonstrate that with a bit of effort, the process is viable. While the stretched hull retains some of the look of
the original hull, some tweaking will be required to sweeten up the hull lines.
Figure 1 - Original frames and new frames added at 21" spacing
New sections were added to respace the frames to 21" spacing. The new frame offsets were exported to an ASCII file and merged back into the original import file. Original frames 1, 3, 6, 8, 10, 13, 15 and 17 will be deleted.
Figure 2 - Original frames removed
We're now left with 21" frame spacing.
We imported the stock hull and stretched hull statics data into our Excel hull balance spreadsheet. This is where
the real fun begins ! It's clear from the hydrostatics data that we're working with a totally new hull which will
have to be completely rebalanced. We'll post more about the balance spreadsheet after we've sorted it out.
Optimum Frame Spacing 25-FEB-2013
Here's a sample calculation of how we can check for the optimum frame spacing using Dave Gerr's scantling rules. The
Scantling Number is derived from the boat's LOA, Beam and Depth at Midship :
Hull Balance & Center of Buoyancy
"A boat has a balance point called center of buoyancy (CB) that is comparable to the pivot point on a teeter totter. The CB can be determined by using figures obtained from Simpsons Rule; the volume on either side of the CB must be equal. But the CB must be located at the ideal position to carry the loadings. The approximate desireable location of the CB is obtained from published figures or percentages of the waterline length gained by experience. The CB is virtually always aft of the mid-points of the waterline length. If the calculated CB is not within the design parameter its back to square one and modification of the lines.
But the designer isn't finished. Weights have been estimated and must be finalized. The weight of every component of a boat must be calculated and it's distance from the CB noted. Consider what is involved. A wooden boat may have a 1 14" x 3 1/2" keel 18' long. The volume of the member times the weight of the wood for a given volume is calculated and the center of that weight in relationship to the CB determined. And this is done for everything that will be in the boat. If the total weight isn't equal or close to the displacement or the weights are not equally distributed about the CB, modifications will be necessary. An experienced designer gets an "eye" or "feel" for a set of lines and can usually finalize a set of lines with minimal revisions. A neophyte designer may spend days or weeks bringing all of the factors into balance."
Glen-L WebLetter # 3 : What's the difference between "drawing" and "designing" a boat ?
How to Balance the Hull
Hull balance is discussed in Glen L. Witt's book
Inboard Motor Installations . This info is very helpful if you're trying to resolve a handling issue
and you don't know where to start.
To simplify the balancing problem, here's a loose quotation from the book where Glen compares it to a child's teeter-totter :
- Calculate the weights. In the example, engine, passengers, fuel
- Choose a starting point. This is called the datum. This point is arbitrarily placed at the
- Measure the distances from the datum to the center of each object. In the example, you must find the
distances to the center of the see-saw and each of the two kids. If the see-saw is 16 feet long, the center is 8 feet from the
datum. The kids are sitting exactly one foot from the end on either side, so their distances from the datum are 1 foot and
15 feet respectively.
- Multiply each distance by the respective weight. This gives you the moment for each object.
First, the see-saw : 30lb * 8ft = 240ft*lb.
The first kid : 40lb * 1ft = 40ft*lb.
The second kid : 60lb * 15ft = 900ft*lb.
Add the moments to get 1180ft*lb for the total moment.
Add the weights of all the objects. The sum of the weights are 30lbs + 40lbs + 60lbs = 130lbs.
Divide the total moment by the total weight. 1180ft*lb / 130lb = 9.08ft. This is the distance from the datum
to the center of gravity.
Source : Inboard Motor Installations by Glen L. Witt and Ken Hankinson, Chapter 2, Motor Location and Hull Balance, page 13
Way too much info in Chapter 2 to summarize here, so ordering the book will provide more insight.
Here's a link to an article by Glen L. Witt which defines the hull's center of buoyancy :
CB... What the heck is it anyway? (WebLetter 25)
Shaft Layout Diagrams 29-OCT-2013
Now that you've stretched the hull, the next step is to determine the optimum engine placement and propeller shaft angle for your
installation. The link below displays shaft layout diagrams for typical flywheel-forward and flywheel-aft installations :
Shaft Layout Diagrams