Transom argument

[jammer]

Can one of the design Gods settle an argument for me? Discussing an outboard boat with a 10" hydraulic jack plate, 250 HP, my contention is that the degree of transom is inconsequential as far as the hole shot goes.

The engine has the trim motors on it's mount so the arc the motor travels would be the same whether the transom is at 15 degrees or 19 degrees. My buddy disagrees. Am I in the ballpark on my reasoning?

Thanks ever so.

[marshmat]

Let's assume for the moment that the outboard in question has a trim range of -5 to +15 degrees while running at speed (more or less typical). Let's also assume that its bracket was originally engineered for a 5 degree transom (also more or less a typical value).
On a transom raked back 5 degrees, you can thus have the prop pointed anywhere from -5 to +15 degrees from the horizontal at rest. (Not counting the tilt-up mode here.)
Rake the transom back 15 degrees and you now have a travel range of -15 to +5.
Rake it back 19 degrees and you have travel of -19 to +1.
Pointing the prop that far down seems rather ridiculous as the thrust vector would likely pass well above the boat's CG, especially with the big setback jack. I don't see any point in angling the transom so sharply.
The driver's skill with the jackplate and trim controls, and the ability of their motors to respond quickly, will have a vastly greater effect on holeshot.

[jammer]

Thank you Matt.

[Willallison]

It's never quite as simple as yes or no....

I'm not sure where Matt get's his 'standard 5 degree transom' from - I always considered 12- 15 as being the norm....
But that aside, it rather depends a lot on the boat in question. If, for instance it were a heavy, overloaded, underpowered pig, then the increased transom angle would allow you in effect to trim the motor in further, possibly - though not definitiely, improving it's ability to lumber onto the plane.
Conversely, at high speed, a 19 degree transom would allow an inexperienced operator to trim in to such an extent that some serious dynamic instabililty issues might arise...
For you average boat, I'd agree with Matt - the operator would have as much effect on performance as anything else.

So - answer?..... depends...

[hmattos]

If it is any help to complicate the issue, here in the UK there is a training philosophy of " Trim right in before you accelerate" This comes from the water skiing users who need a good acceleration and then want to keep the bow down during the acceleration phase, even with a tow load on a high ski pole.
We build fast RIBs, see www.explorermarine.co.uk, often used for skiing and we have revised designs over the last 10 years to increase transom angle from 5 degrees to 10 to 13 and now we build at 15 degrees. This gives an excellent acceleration from rest with the engine trimmed in and yet the modern engines can still deal with running trim to cope with sea conditions and passenger loads.
Further, most of our current designs have fully moulded transoms which extend BEHIND the engine to extend the planing surface and thus keep the bow even lower during acceleration which much improves " hole shot" performance

[marshmat]

Raking the transom back approximately 5 degrees is more or less the norm among most outboard-powered boats at dealers around here. I don't think it's actually written down anywhere, it's just what most of the outboard brackets seem to be based on, at least in this area.
If the motor's trim system can handle full thrust at what is, from its perspective, a fairly high trim-out angle, the 4-degree difference probably won't be that significant.
Will and I seem to agree that it's probably going to come down to the operator. You have a jackplate, power trim and throttle. This gives an enormous degree of control over the thrust vector. The helmsman has to know how each of these controls will affect the boat, and how they interact with each other, or else it'll be a dog no matter what.

[Willallison]

I'm a bit confused Matt - I've never seen anything other than 12 degrees specified, as per the pic below, for both outboards and sterndrives. I would have thought that with this as the industry standard, you'd have a stack of boats in your part of the pond that can't trim in sufficiently...??

[marshmat]

Maybe I'm just getting my numbers mixed up then.... It wouldn't be the first time....