# inboard geometry

Discussion in 'Inboards' started by epoxicologist, Apr 17, 2019.

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1. Joined: Apr 2019
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### epoxicologistJunior Member

Does anybody know about motor position, in the prospective of "center of gravity"
How motor position is determined for inboard direct drive?sn

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### Mr EfficiencySenior Member

It has to be considered along with all the other weights, in the total equation. But obviously there are considerations of shaft angle, for example, which may mean you don't want it too far back.

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### gonzoSenior Member

Shaft angle may also determine the need for a V-drive or an offset transmission. Either one can be used to position an engine further aft.

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### epoxicologistJunior Member

Thank you for your response,The boat associated with the "Inboard motor position calculation"I'm requesting help with
is a 24' deep-V originally stearn drive I/O (Merc, alpha).So any other drive system would be a more foreward application.
in an aproximation in consept , in trying to match the proportions (motor position) of say a mastercraft ski boat,
Its motor seems to sit almost mid hull but slightly to the aft my "guess" would be middle of hull being 50/50 bow/transom.
Mastercraft type boats appear to be about 60/40 bow/transom for motor. 60/40 on my boat means a foreward shift of aprox.
8' and with a turbo 400 transmission in a direct drive application , still leaves close to 5' for propshaft.
My consern is i don't want find that I' placed it to far foreward and bow heavy.

Any insight as to correct hull balance as to engine placement is greatly apreciated
thanks again

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### Mr EfficiencySenior Member

24' is a fair size planing hull, a single engine is not going to be huge proportion of the total weight, so it gives some flexibility as to placement. If the boat has needed tabs, it might benefit from a weight shift forward. The thing about planing hulls, is that the leading edge is the area of highest dynamic lift, and if the bow dips, the leading edge goes nforward, and the centre of lift with it. In other wordss, there is a fair bit of tolerance, but a lot depends on what you cab do with other weights as they are presently distributed, like fuel tanks and what-not.

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### kapnDSenior Member

Ski boats are set up very specifically to tow skiers well in calm water, not a good basis for comparison to a 24’ deep vee hull that is intended for use in open waters where a variety of conditions will be encountered.
Looking at similar rigs and how they perform would be the first step.
Unless you’re going to do major rearrangement of the interior layout, I’d recommend a vee drive.
Don’t forget to plan for steering, raw water and exhaust systems that will need to be added in the absence of the stern drive.

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7. Joined: Apr 2019
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### epoxicologistJunior Member

Thank you for responding,
And yes, I'd say i qualifies as a "major rearrangement". I've owned the boat for 15yrs
and spent a small fortune in out drive repairs & even more on replacement of complete Mercury Alpha One I/O's upper and lower
through the years . It's an old fiberglass boat thats seen more hours than most boats do before a last or final voyage.
So it's had issues, but I love the boat , and stayed on top of mechanical issues the one neglected need it has is after
50yrs it needs so much of the integrated wood parts be replaced that I decided to do a complete structural and or wood
components repair/replacement,Stringers ,transom ,floor engine mount members everything,Fiberglass from my understanding,
doesn't degrade unless damaged.I'm making a 50 yr old boat essentially new, and have a blank slate to redesign any way I choose.
so I'm modernizing systems to current tech. and hydrodynamic performance concept design.
(Direct Drive shaft to Surface piercing prop recessed in Vented tunnel pocket).Gas tank and engine swap location
The V-hull is so deep at new motor location (slightly behind driver seat) that A big block GM complete doghouse is only about a 14"
rise above floor instead of the original 30" at transom.
reason for converting is 1) parts cost,I can replace a shaft ,prop ,and rutter 3 times for the price of a Mercury I/O
2) Direct drive shaft is more efficient than I/O
3) shift forward should promote better balance ,It seems so *** heavy when gets any airtime
4) If I'm gonna give up the same 60 HP that the I/O or V-drive eat ,I prefer 3spd auto trans, 3rdgear ratio being 1:1 w/engine.
5)Vented Tunnel allows z surface piercing prop to relieve the vacuum of 0 speed take off until planing speed is reached
6)Surface piercing prop studies have shown as much as a 30% improvement in over all performance compared to a fully submerged prop.
this is the logic behind this concept design based on what I've learned through various articles and knowledgeable people.
Thanks again for responding,
does my theory and design have any flaws or misinterpretation on my part ?
Tell me what you think,I love 2nd opinions and not afraid to admit when I'm wrong.
I'm finish commercial carpenter 33yrs ,not a boat engineer.

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### philSweetSenior Member

Robalo? Stamas? Grady White? I'd be surprised if there's a 50 yo Wellcraft remaining that is straight enough to take a big block. What is it and what does it weigh?

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### DCockeySenior Member

Planing boat performance is sensitive to CG location and also prop shaft angle. I don't know enough to offer much advice. I do know enough to say that install the difference with a big block engine forward enough for shaft drive compared to a stern drive along with the change of prop shaft angle may cause a significant change in the behavior of the boat. A simple analysis based on Savitsky's method by a knowledgeable person should provide at least an indication of how the modified boat might perform.

A shaft drive eliminates controlling trim angle by changing outdrive angle. With the CG forward the boat will run flatter, and trim tabs can't be used to bring the bow up, only to lower it.

Do you know of any similar conversions which have been completed? Do you have any knowledge other than owner reports about the results?

This sounds like the boat equivalent of an automotive hot rod project. A few hot rods are nice vehicles that the owners enjoy, others are driveable but if the owners are objective (difficult to be after putting considerable time and money into a project) are disappointments, and a considerable number are unfinished, sitting in the garage or under a tarp in the driveway.

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### YellowjacketSenior Member

You really need to rethink moving the motor that much. A big block weighs almost 700 pounds, add a transmission and accessories you are closing in on 1,000 pounds. You should calculate the CG shift and you will likely be very surprised. And don't use fuel as counter ballast, as the fuel burns off the CG is going to move forward. Your hull was designed around an aft mounted motor and that is where you really should keep it. That much change in CG is going to make the boat bow heavy and there are a number of issues that could surface. If you want to move the motor that much you're going to have to do a lot more than just moving the motor. Also there is no reason to put an automotive transmission behind the motor, running an automotive transmission in a lower gear at all times is not going to be a reliable solution. You're asking for the transmission to do things it was never intended to do. If you want to go inboard and need transmission get a proper marine transmission. If you really don't like the current drives it would be a lot better to replace them with newer, better drives, or replace it with a V drive. That would keep the CG in place and be a lot easier overall. While you can do what you are thinking of doing, it doesn't mean you should. While the boat will not be worth what a new boat is, you're going to slaughter the resale value with a cobbled up drive train.

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### DCockeySenior Member

I am not familiar with ventilated tunnels? Where will the air to ventilate the tunnel be supplied from? How will the front of the tunnel be configured to ensure water flows cleanly into the tunnel? Do you know of other examples of ventilated tunnels in deep-V hulls?

You mentioned the engine position would be similar to a Mastercraft ski boat? Do those boats have deep-V hulls?

Moving the engine forward is likely to cause bow down trim when at rest or moving at slower speeds.

12. Joined: Apr 2019
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### epoxicologistJunior Member

It's a 1970-71 24 foot Fiberform (brand/manufacturer) Catalina (model)
It has 7 feet of cuddy cabin with a ceramic toilet concealed by original dining table top off the post insert as support for cushions
enclosed by a folding "saloon" type door w/a flip down cap dividing the dash ,and 16 feet open seating space after the cabin doors.
and the sales promo brochure shows a double occupant passenger seat at the right side at the cabin/dash w/ a externally facing fridge in the right cabin panel.
never have I scaled it,but most opinions Ive got through the years is between 3500-4000 #'s
its capacity rating says a max of 2500# people supplies & gear.
And the F,glass is in great shape if ignoring the Gel-coats appearance. Needs a good paint job.

13. Joined: Apr 2019
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### epoxicologistJunior Member

No Mastercraft is a flat bottom w/scaggs. J was only using it for an example to visualize the propulsion I'm considering.
And the ventilated tunnel would have a pair of tubes at front of the tunnel pocket and in front of propeller that rise to a point that well above water level when at a stand still float.It lets the prop vacuum open air into the tunnel immediately relieving drag from floating still up to planing speed and lets the RPM's build quicker
I'll probably plumb mine to the transom and vertically to a ported luver cap just above the rubber bump rail @ shell to hull seem.
The tunnel is a half radius big enough for clearance of half the diameter of the propeller and following the planing suface into the tunnel
the propshaft should terminate at said planing line with the prop splitting water 50/50 when up to planing speeds.

14. Joined: Apr 2019
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### epoxicologistJunior Member

Thats a reasonable prospective worthy of consideration and I appreciate the input,
The only part of what stated that realistic is compromising it's resale of an altered drive system creating retarded performance manifested through a cobbled propulsion system,
That statement implies a lack of quality workmanship of an unresearched idea engineered in a"shooting from the hip" disposition
This is , and will not be the case. And I'm not saying that your wrong,9 out a 10 backyard mechanics modified results on whatever could be they're choice in projects
would be what I call a "roach".due to a lack of understanding what they're proposing.
But thats why I'm online to gain a competent prospective consulting with people like yourself to ensure that it doesn't end up a cobbled up boat sitting in the driveway w/no resale value .
you make a good point, And I haven't made any permanent changes .That would be making commitment in the wrong direction. untill I know what changes I make are not wrong for the boat,I won't feel right about
changing anything. thanks for responding

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### Mr EfficiencySenior Member

Huge job, even without you idea to invent a novel drive system. And as someone mentioned above, you have something that would basically be unsaleable, after all your hard work. I don't know anything about the hull, but there are precious few old glass hulls that are worth such restoration projects. They need to be outstanding hulls. If it was an old Formula 233, I'd say worth a rebuild. But no exotic drive systems ! And it is all a guessing game as to what a radical change in weight distribution, and this new-fangled drive idea, will add up to. Too much work, too experimental in several ways, I would not do it.

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