Donzi Hull modification

Okay, so my other project is a 16' Donzi "hot rod" of sorts... I'm playing around with some of the modifications racers use to increase performance. But before getting out my tools and just hacking away, I'm considering alternate methods to achieve the same goals.

Many performance boats raise the x dimension to reduce drag and gain speed. A common practice with this modification is to also to add a "standoff box" or an extension of some sort to the drive to compliment the higher X. Pretty easy with an outboard... not so much for an I/O.

I'm not an engineer, NA or even a technician... just a backyard hack . My little Donzi will be running an Alpha One I/O drive. Alpha drives once came in a 2.75" shortened version called an Alpha SS to mechanically raise the X. Unfortunately, these SS drives are no longer available. Further, I have never seen a standoff box made for an alpha, although there used to be a device called a "stern jack" but, these devices have issues of their own and are also NLA. (extension boxes are also a HUGE $ investment)

So, I have shortened a regular Alpha drive casing 3" to emulate an Alpha SS and effectively raise my X. Now, I'm looking to recreate the effects of a standoff box without re-engineering one. My idea to do this is to notch the hull at the transom which in effect extends the relative distance of the prop from the functional transom/bottom edge of the boat.

Work is already being done to the hull (transom/stringers replace) and I was thinking of adding a PAD on the hull which is a different issue, but modifying the hull is very do-able when the boat is in this state.

Question(s): Am I crazy? Will this work? What handling characteristics will be affected, if any?

Thoughts?

Note** drawings not to scale

 

What are you trying to achieve overall? Just higher top speed? Different handling?

I've never had any difficulty pushing my 1973 Donzi 16 to speeds that were clearly approaching unstable/unsafe with the stock I/O configuration and just a bunch of extra horsepower and the right prop. I'm in the midst of changing engines again, but had a ridiculously overpowered 302 in it for quite a while and could never use all the power. Dropping down to "only" 340 HP engine now.

And as an aside...that Alpha is a pretty weak drive; what kind of power are you planning on putting through it?

 

I'm trying to achieve more... with less.

I'm building a 340-ish HP SBF 347 stroker for this boat. This boat is a "hot rod" that is being modified as a benchseat for fun & show with the inspiration of the Cowes-Torquay class 3 racers of old.... (similar to Wyn Mill II and Souters built boats like Thunderflash & Thunderbox). It will be driven but, not raced.

I want the boat to look the part and perform well too... all the while, not destroying the every day driveability. Target is for a 70-ish mph boat but, I will not be disappointed at 65, or even 60mph. The Alpha is light and in it's shortened version is much stronger than the "stock" long design (although stock is more than enough drive for 340HP). I am locked into the SBF (original configuration) and do not want a high strung race engine... just light and reliable.

Walt Walters originally shortened the strakes from the prototype boat to cure other issues with the original 4 cyl. Volvo engine/drive. These boats barely hit 50mph while C16s have since been documented over 100mph. There are plenty of well documented 70mph Donzi 16's around too, if I wanted to copy them.... but, this boat will be different. I will be elongating the lifting strakes as Donzi later did to cure the "chine walk" issues (in the mid-high 50 mph area) found in the early hulls used with big modern power. I have not decided on a pad yet as this may be too extreme for everyday use.

So, do you think the above described hull mod will work to imitate an extension box?
Do you think it will change the handling?
If so, how?

 

Your experience with the Alpha drives must be quite different than mine; I could never get them to hold together very long with anything over 300HP. Even adding external cooling and remote oil reservoirs.

Good luck with that part; I blew up plenty of other things over my years of playing with go-fast boats and it didn't stop me either.

Were it me..I would add the "box" rather than shorten the bottom. Is that a "non starter" because of the class rules you are generally trying to adhere to?

Raising the prop line and reducing prop immersion much is going to adversely effect one of the more famous characteristics of the 16..the ability to carve hard turns so well. In my humble opinion.. I have a Champ 17 that demonstrated that pretty well and I ended up putting a hydraulic lift bracket on the back to be able to better control prop immersion for straight high-speed runs versus turns. That was with an outboard, of course,..a warmed up Evinrude 235 pushing about 270 HP.

 

The longer housing (and therefore longer shaft) flex too much under great torque. The shorter version does not. I'm told, it is also possible to retro fit larger bearings etc. Most of these issues are from not coming off throttle while airborne.... Well it happens but, probably not often the way I drive (like the old guy I am) .

It's actually harder to make a box.... or my own "stern jack".

An acquaintance in Vancouver has an 18 2+3 with both the Alpha SS and "stern jack" installed. Performance with a stock 260 HP SBC is "heads & tails" above the stock alpha set up in all respects. I'm convinced the set up works. I am looking for an alternative way of getting to the same place.

Both are 7x24 boats (7' beam, 24* deadrise) but the 18 is 16.5" longer... all ahead of mid-ships. We are just talking a 3" difference on the X. Maybe a 4' high x 8" deep notch out of the very base of the transom.

Why do you think the handling will be compromised?

 

How goes the hot rod project?

Why not just modify the hull to get everything you want?
Change "X" dimension.
Create a better "Pad" with modern design for speed and stability.

 

If I move the x up 2-3 inches I need to build a "dog house" on my engine lid and it also raises the cg.

I have decided to change hulls (for other reasons).... I'm looking at a Formula Jr. or, a scratch build from wood to my specs.

The drive is coming along well. However, I will not be able to really tell until it is installed and wet.

It's a project... and not a cookie-cutter at that. These things take time

 

Don't move the engine/drive up, move the bottom of the hull down. Make a pad/platform that is stepped like in Fig. 3. but with the bottom of the pad where it needs to be for "X" to be right.

Seems like HydroStream had the right idea but didn't execute very well, the pads were too narrow to be stable. Duplicate a current pad from an Allison or other such successful design.

 

If you're suggesting to add a false bottom.... that's interesting. I'll have to think about that for a while.

 

Not sure if "false bottom" is the proper terminology.

Flatten out that deep V from about mid ships sternward.

Imagine an 8' long triangular shaped piece of plywood. Pointed on one end and say 18" wide on the other. The pointed tip touches the keel about mid ship and extends rearward and downward so the 18" wide end is spaced down below the hull at or ahead of the transom for your desired "X" dimension. Fill in the sides so it becomes a wedge looking pad.

The amount of deadrise on some of those old Donzi's was incredible. I bet they rode plushly.

 

Like this.









Only wider for better stability.

To go fast you have to improve your power to drag ratio. Either increase power or decrease drag.

You will need to have some sort of low water pick up for engine cooling as well.

 

Indeed they do ride nicely. And boy can you carve a tight turn in one too...