I/O to Inboard Conversion; DWL Question

Hello All

I am new to this board, first post, so please go easy on me!
My question is mainly for Naval Architects/Marine Engineers with experience in planing hull recreational boat design, although I encourage anyone with an opinion to reply.... When you design a particular hull that can/will end up powered by inboards, outboards or I/O's how do you deal with the DWL since each of the engine installations will have a different CG/CF, Do you
1. Ballast the hull so that the DWL is the same for all three cases
2. Redefine the DWL so that it is different for all three cases
3. Do a combination of one and two (add some ballast & redefine DWL)

The reason I've asked this question is my two brothers and I bought a 1969 28' John Allmand for really short money that was converted from twin I/O's to twin conventional shaft Inboards. The conversion was half done when we bought the boat plus it needed some cosmetic work and a lot of systems work. The bottom line is when we put the boat in the water she sits a little (3-4") nose heavy. Of course the only reference we have is the existing boot stripe which I'm sure was applied based on the I/O configuration. Under power the boat gets up on plane fine and seems to handle Ok although we did not get to put a lot of hours on the boat due to port engine troubles that we were unable to square away (next year!). One of the problems the nose heavy attitude creates is bilge water collects in the bow and none of the cockpit drains (they're all aft) work when the boat is on the mooring or otherwise stationary. We did add a third bilge pump forward (had to cut a hole in the cabin sole) and we will add forward cockpit drains this winter as well.
My problem is I'm trying to decide whether to, ballast the boat (by experimentation 3 guys on the swim platform levels the boat, that's a lot of weight with a decent moment arm), repaint the bootstripe to the new "scum line" or do a combination of the two.
Any help or suggestions will be greatly appreciated!

thanks, MDUFF

 

Adding ballast to a planing boat must surely be an act of last resort. You will simply make the boat slower and less efficient. Try moving as much existing on-board weight aft....things like batteries, food stores etc are relatively simple. Things like fuel and water tanks can be harder to do, though well worth the effort (at least while they're full....)
As long as the boat doesn't misbehave - particularly at speed - then I'd move the boot-stripe. maybe throw something heavy down aft when the boat's moored to prevent water collecting where it's not wanted...

 

Thanks

Will
Thanks for the reply, that is the answer I was hoping for. We are already working on dealing with rain water and bilge water in unwanted places and will be installing a holding tank and a potable water tank as far aft as possible (right against the transom). If we evere get our hands on a generator that will also be mounted aft. Fuel tanks were moved aft when the conversion was done.
Since I have received only one reply I'm guessing that this is either not a very interesting topic or I have posted in the wrong sub forum. Can you shed any light on that?

Thanks again, MDuff

 

First off, every straight inboard I have owned had the bilge forward. Most recently My 28 Marinette did. I suspect a John with original inboards did as well unless...... It used Vdrives. I would say that would be the true fix. That would place the engines in the stern where they sat originally yet retaining the inboard setup. Probably would be a little faster, bow would ride a little higher but controllable with trim tabs. This would be preferable as you could adjust better to suit load

 

Thanks for the reply Woodboat.
Converting to V-drives is not in the cards right now (time and $$$).
If I read into your reply a bit it sounds like you agree with Will in that the nose heavy attitude of the boat at rest is not that big a deal, which implies that repainting the boot stripe along with moving as much existing weight aft as possible is an acceptable compromise. Is that a reasonable interpretation of your post??

Thanks again, MDuff

 

Pretty much.I would add had it been designed originally as an inboard the bilge probably would have been forward and weight already aft. Every inboard I have owned had the fuel tanks within inches of the transom, with water and waste flanking them. Batteries are usually in the genera vacinity as well. Are you getting plenty of lift on the bow under power?

 

With the limited sea trials we were able to do the boat seemed to come up on plane fine, did not porpoise and did respond to trim tab adjustment. Even at low displacement/headway speeds the bow comes up and the stern squats. Whether or not it matters all of this testing was done in a protected harbor, we have no open sea time on the boat at all.... next year.
As stated earlier, the two 85 gal. fuel tanks are aft although not at the transom. Two 30 gal. water/holding tanks will be installed this winter aft of the fuel tanks, right against the transom. The four batteries are along the centerline of the boat in between the fuel tanks.

This boat model, the Allmand Ranger 300, was available in a number of configurations; soft top, hard top, fly bridge express and sport fisherman. Our boat seems to be the sport fisherman which has one helm on top of the cabin all the way forward with a full waist high fiberglass surround and a full chest high aluminum framed glass windshield that wraps half way around the the fiberglass bridge surround. When I'm standing at the helm I'm well forward of amidships pretty much at the max beam of the boat. This configuration puts a lot of weight forward by design, the worst case of any of the models available....... So maybe it's not such a stretch to think that the bilge forward is not really a problem at all.
That last paragraph is just me thinking out loud/rambling. LOL!

I'm working on a 2D drawing to help me visualize the weight distribution, I'll try to post it when I finish.

Thanks again, MDuff

 

Placing all your tankage as far aft as possible is a very good idea. This will improve your "at rest" trim and planing attitude. The tankage you are indicating will be more than the 3 guys on the swimstep.

Another good old rule has to do with the rolling moment of the hull... if the weight is placed to the outside (gunwales), the rolling moment will be longer, if placed towards the centre, the roll will be shorter. Sounds like you're going in the right direction. Good Luck!