converting 34ft twin inboards to outboards

Anyone have any experience with or suggestions for converting a 34ft+ inboard powered boat to outboards? I have a 34ft Silverton (12.5ft beam) that was powered by twin inboards. I would like to power it with two 150hp or smaller outboards. My one concern is that I need to design a bracket for the outboards, and I believe I need enough bouyancy into the bracket to insure that when the boat pitches bow up, the outboards do not submerge. Also, I do not intend to power this boat with enough hp to plane - I'd like to be able to calculate how fast it will move with various engine sizes. I would like to get a cruise speed of 10 kts.

 

1st question ... why?

 

Will,
I need to get thru shallow water where I'd like to raise the engines. I also like the ability to get all the metal out of the water, and easier maintenance.

I live on the water and I want to pull the boat out of the water when hurricanes come our way, and this will be easier to do at home without running gear hanging down.

I bought a boat that the previous owner had tried to swap the gas engines for diesels. It has V drives that didn't fit, shafts and struts that are shot, rudders broken, prop shaft seals bad, etc. - to put inboards back in all of this would have to be replaced, so this would be a good time to switch to the outboards.

I don't have to travel far to fish or dive, so I don't need a fast boat, and I don't want to spend a lot of money on engines. The new 4 stroke outboards also have less stinky exhaust.

 

Ok - well a couple of points.
Planing hulls tend to make poor displacement boats, particularly in rough weather. But, so long as you don't try to push it up towards it's displacement hull speed - probably about 7 knots in your case - then it will be ok in calmish water.
For that you will not need anything like a pair of 150hp engines. Without working it out, I would think that a single 40hp - preferably one of the 'big foot' varieties - would do the trick.
The biggest problem you will find is that because you are so restricted in the diameter of the prop, you will suffer from a lack of 'grip' on the water. Coming alongside a pier will require hitting reverse several boatlengths out etc. But that just comes down to practice.

Of more importance is weight distribution. You will be pulling around a ton out of the boat and replacing it with ony a couple of hundred kg - at the most. As a result, the boat will sit higher in the water - possibly lifting the chines clear. This will make it tender at rest (rocky-rolly) and noisy as waves slap the underside of the chines.
It will definitely require some movement of other onboard weights and probably the addition of ballast to bring the trim back to it's correct position too.

 

Will,
Hmmm.... I remember reading about displacement hull speed, but my crainial neurons no longer can retrieve that info... Can the displacement hull speed be increased with any (relatively) easy modifications? What is a fast displacement hull speed for a displacement hull boat (just for comparison)?

 

silverton outboards

Anotherthing to take into consideration is the length of the shaft to get below the silvertons vee but the biggest prob is the boat. That transom was not designed for outboards and the stringer system needs to be beefed internally. If the transom is to thin or possible delaminated it has to be fixed and reinforced with the propper amount of cloth. I'm not saying that you cannot do the work yourself, but get several expert oppinions before you start.
captword

 

short answer: no
displacement hull speed is equal to about 1.34 x square root of the waterline length. In a hull like yours it is likely to be somewhat less. The only way of increasing the hull speed is to make the boat longer (or a whole lot narrower). I wouldn't try to push your boat more than about 5 -6 knots

As far as beefing up the hull goes, you may need to do a bit, but the smaller o/b that I'm suggesting I wouldn't think so

 

Thanks for the help. I want a boat that I can take offshore - to the Bahamas from the Florida Keys. Sounds like this is a bad idea - trying to convert a planing hull to a displacement hull boat. I appreciate your responses.

 

It could be that you have other options. You say you have 2 diesels that are running but the vdrive transmission combined is shot?

You could turn the engines round and fit a pair of jets. Same price roughly as a pair of new outboards.

 

I've gotten rid of the diesels - sold them. I really would like outboards as I do take boats into shallow water - I park boats in two lagoons that can get less than 3 feet depth. One is on the bay side of Key Largo - the wind blows from the east, and I have about 2 1/2 ft. of water. The ocean side of Key Largo - at hi tide, I have 3 feet. All other times less.

The Silverton 34C has about a 5 ft hi transom - it will definitely need to be reeingineered and reinforced, and I can get help with that.

Iya in Jakarta posted info on his conversion of a 29 Luhrs from 275hp diesels to 250hp outboards and it worked very well - faster on less horsepower and it corrected handling problems with the Luhrs. He says that they've done this to a lot (12+) boats in Jakarta.

I was thinking of having one of the bracket manufacturers build a custom hull extension/bracket for the boat. I've talked about this with one of them (forgot which one - have to look at my notes). They were proposing to extend the hull 30" - this would give some floatation near the engines that would keep them dry(er). But they have not built a bracket for a boat this size.

 

I think that this might be a good time to look up the fuel consumtion of a 250 Hp outboard running at full. Please be seated while reading.

 

I recall many moons back, Bertram released versions of their 25 and 28 foot boat with OMC's Seadrive's. These were basically just outboards with their own bracket designed to bolt onto the transom. The boats were much lighter, considerably faster - and yes, more economical too. And that's way back in the days's of very thirsty carburetted 2-strokes.
They did suffer from being tender at rest as I suggested before however...
You really need to decide what sort of boat you want. Whacking on a couple of big, modern outboards will almost certainly give you a faster boat - but if you are after a good rough water, displacement boat, better to buy one that's designed to be just that.

 

Hmmm..... I want a boat mostly to fish and dive locally, don't need to get out to the reef in any hurry (10kts is fine), I'm a working guy so I don't want to spend too much for the engines, the more I spend for the boat, the less I have to spend on fuel....I'd also like to take occasional trips to neighboring islands - the Bahamas and Cuba when they open it up - these are 200-300 mile round trips across open ocean.

My desire is for an economical boat - but I like the size and layout of the 34 Silverton. It is a comfortable size for us to fish and travel on. If I can put some outboards on it and run them slow and burn less fuel, I'll be happy. I know if I put 2 x 250 hp and run them fast, I'll need to get a second job to pay the fuel bill.

 

Can a boat designed as a planing hull (Silverton 34C) be used as a semi-displacement hull boat? If I were to put less 300 hp or less on this boat (inboard or outboard) would it be a safe boat to cruise offshore? What problems should I be concerned about with this setup?

 

In saying this, I'm assuming that your boat is indeed a planing hull....
The problem with most planing hulls is thay they are pigs at semi-displacement speeds. They tend to 'bog down', poking their noses towards the sky and generally wallowing around. It's not always the case, of course, but those that do perform well at slower speeds tend to be light and feature very low deadrise at the transom.
Maybe you could post some pics of the boat - in particular that which lies bebneath the surface..