spira veracruz panga

I will be building a spira 18' panga his plans. I have access to a v-8 vdrive setup that i am thinking about using. According to the study plans this hull is rated to 75 hp max, I know this vdrive setup is gonna be overkill by alot. my question is what is all used to determine max HP.?

 

The designer determines what the maximum power will be. Usually, it is a conservative figure. However, a V8 will probably be three times the power and way outside the safety limit.

 

I agree with what you said gonzo. but i also know that not all boats that have big Hp are 28' CC constellations either. That is what is driving my question. I know that the vdrive setup that i have is to much but i was looking into the future, Ie. my next build

 

Experience over many different kinds of boats in varying conditions determines what power is appropriate, there are formulae that have been created to give a practical guide. But for a planing boat, if you simply multiply the waterline length by the waterline beam, multiplied by the draft (not counting skegs etc), in feet, you will have a "rough" hp guide. For example your 18 foot panga might be 16 x 5 x 0.75, = 60 hp. A boat that might be suitable for your V8, say 19 x 6.75 x 1.25, = 160 hp. The latter would be a deep-vee around 21 feet. Accurate measurement of the draft is the key to that.

 

rigged draft or empty draft???

 

Loaded draft. Obviously it is not a "scientific" method, but a rough approximation that works OK. A boat with steep deadrise draws a lot more than a flat-bottomed boat, so you can see it does allow for that, to some extent.

 

The weight of a V8, trans and V drive will far exceed what was intended. This alone will discount the practicality of the idea. An average 75 HP outboard will be in the 350 pound range, but V8/V drive will triple this figure, which on a boat of this size and configuration, just isn't reasonable.

Power requirements can be (typically are) calculated, not guessed at. These calculations can often present more HP than desirable on certain hull shapes, like the Panga's. CB and CG concerns will become problematic as well as pushing the boat into a longitudinal instability realm, that can be very dangerous too.

Reconsider your modification plans.

 

PAR i am glad you chimed in..i am not gonna put the vdrive in my panga..let me get that out first!!dumb not crazy lol..i fear with that much dead rise the chine walk would be nuts. I was thinking further in the future..another build

 

Seriously ? From your opening post, it could be easily be deduced you were looking to use that engine, in that boat. And as for chine walking with "that much deadrise", that really would not be the problem. There are a few 18 foot boat designs that could happily take a V8, ( mid-mount ski-boat, e.g.,) but your panga has not been conceived with that in mind. Even if you could place a V8 in the boat without throwing out the weight distribution, you would be all dressed up with nowhere to go, as there is no way the hull could cope with the speed potential safely. So if it can't use the power, it is pointless to have it there, you are just carting around lots of weight that will increase the planing speed threshold, and limit what load you can otherwise carry. It is unequivocally an outboard boat.

 

I hear you, but the OP can probably get a reasonably accurate gauge from that "bush" calculation I mentioned. It certainly ruled out the V8 panga, even allowing for the draft increase.

 

I have a few V8 powered (outdrive or jet) designs, but it's not an easy configuration, as it eats a lot of room in the boat and the weight requires a pretty fat butted design. Now, with modern V8's which are 40% lighter than fully dressed first gen small blocks, this is changing a bit, but still well over what an outboard will enjoy. An outboard on a bracket eliminates any in cockpit footprint, plus it's weight savings.

Maybe it would be best if you fully defined what you're looking to do Fiddler.

 

with the panga i have an OMC 60hp thats going on it..Yea i had thought about the vdrive for it but not long .as far as the vdrive in the future ill be building something that will do it justice, and was wondering what the thoughts are for a decent runnabout build. If i messled you guys i am sorry

 

On small boats, straight shaft setups suck when it comes to maneuverability. They eat a lot of cockpit volume and though fairly simple to arrange, just not worth it in terms of weight, footprint and maneuverability, compared to other choices.

 

i am not concerned with that up here where i live is not that busy ever. .and from what i can tell it takes up about the same room as an I/O setup

 

An I/O takes about half as much length in a cockpit, as a conventional straight shaft and many V drives require a raised sole. Both the straight and V drives arrangements also truly suck in maneuverability, compared to vectored thrust.