Moment to trim question on hull extension

Hi everyone. long time reader first time poster. These forums have been really helpful to me, thank you all.

I am a commercial fisherman planning a 26" hull extension of my 20 ft Larson international. It is the delta conic Hull design by Harry Schoell.

Reasons for extension are:
Converting from a single 115 outboard (underpowered for my current scenario) to twin 115 outboards for both reliability, and to be able to plane with heavier loads.
More aft deck space to accommodate a larger bait tank and have more working area.

I considered a bracket, but the transom needs to be recored so must be removed either way, and I have a plan for the aft deck with the extension that I am quite partial too. I am still open to repairing the transom and tying in knees to support a bracket.

I have calculated the displacement of the submerged portion of the extension at 1028lbs. The weight of the engines is 772 lbs dry weight plus the weight of the extension itself (10 gal polyeter resin 1 sheet of 3/4 inch ply 1 gallon of gel coat. and roughly 20 yard of fiberglass) roughly 200 pounds. For a total of 972 lbs. I figure with the weight of propellers, oil and steering system this puts me very close to neutral buoyancy.

Is this the only calculation I need to do to make sure that the boat trims correctly?

As an exercise I calculated the water plane of the Hull WITH the new extension
WL length: 230.4"
WL beam: 79"
80% of the area for planing power vessels
derived the MT1=542.25

This is where I had to guess I as don't have a any real plans to derive the actually LCB so i used the point where the fuel tank and center console are centered on as the CG, which would be 10.3 feet from the new transom. I know this is a big assumption.

I calculated the trim of the outboards at 16.62 inches!!!! If i am correct (which i am probably not) that means that i definitely should not do this modification.

I thought maybe I should subtract the trim of the current motor from the current hull length which would raise the stern.
WL length: 206" 386 lb motor CG 8.3 feet from transom. calculated to 7.35 inches of trim at stern.
but even when I do that it still leaves me with 9.27 inches of trim at the stern.

I assume I am missing something big here. If anyone has some guidance for me please let me know.

sorry for the long post thank you

 

Welcome to the Forum Danny.

In view of how you only joined yesterday, you most likely cannot post photos on here yet - but when you are able to, some photos of your boat would be very useful to go with your description above.

I have seen many commercial fishing boats here that have been lengthened (or 'stretched'), usually by about 3' - 4', and this has invariably had a positive effect in various ways (ie not just by creating more space).
I do not see why you should have so much trim by the stern if the amount of weight gained in the additional hull structure required is roughly similar to the additional buoyancy that will be created.

 

Bad assumption.

The cg is not at the tank/console location. It is further back because of tbe engine and hull weight aft. Batteries aft would also move it aft.

Most planing hulls have their lcg about 1/3rd of the length from aft or for a 20' boat say 6.5'. This is from recollection and reading; not any professional training.

Redo your trim calc in one foot increments back from 10.3, to 9.3 to 8.3 and then to 6.3' from the transom of existing. Or make some estimates of the weights of internals and the hull from known data and create a spreadsheet and you can probably dial in the boat within inches.

Using a spreadsheet, as long as you don't cheat too much; you can setup the lcg as a variable and get pretty close. Also, see if the boat is level right now or not. I would expect with one in the boat at the helm; it'd be close.

Also, what are the units on mct?

 

Thanks you for replies. I have started playing with moving the cg around and you are completely right, that is the the problem. After a look at "the nature of boats" Gerr says a good estimate of CG is between 60 and 65% of waterline length aft of the bow at waterline. Which puts me right at 6 ft from transom just like you said.

As the boat sits now it is level in the water. I did the calculations for 6ft and 6.5 ft CG and the numbers work out pretty much even for a CG of 6.5 feet. obviously this will give me a change in my calculations for the extension.

I still have 2 questions. How do I apply this current CG and trim calc to my new extended boat? Because I am adding length the moment of the new engines will be much larger, but because I am adding hull weight, and the weight of a second engine as well as buoyancy the CG is also going to move back correct? or will these two things cancel each other out and I should only be focusing on is the buoyancy of the new extension vs the weight added as bajansailor said.

Here's what I thought.

Let me know if Im on the right track. I calculated the moment of the engine on the boat now at 5.8 inches meaning if the engine was removed the stern would rise 5.8 inches.

Then I calculated the moment of the added buoyancy of the extensions displacement minus its own weight using 1 foot behind the current transom as the center of the new added weight and buoyancy (the center of gravity for an object 2 feet long that is aft of the transom) I know this is a bit flawed as the weight of the transom itself will be the heaviest part of the extension but some estimation required here. I calculated the added buoyancy of the extension using the MT1 of the NEW lengthened waterline would raise the stern another 10".

So if i removed the current engine and added the extension, the boat would sit with its stern up 15.8 inches. Then I would add the outboards to the new extension, which have a moment of 16.6 (using MT1 of new waterline) which puts me at about a half inch of trim at the stern. Which would be awesome. I don't know if adding buoyancy aft of CG is the same as adding weight forward of CG, but if it does then i think my extension is gonna work out.

The units are pounds, ft/pounds and inches.

 

My head hurts, trying to understand your calculations above, especially as they are in imperial rather than metric.

Maybe you are trying to make it all too complicated?
How about just calculating the weight of the materials in the extension that will be added, and the extra buoyancy / volume of displacement gained by this extension.
If the extension on it's own effectively floats at the same freeboard as the rest of the hull, then you will not have any change of trim when you join it to the main hull hull.
If it floats higher, then the stern will come up a bit when it is attached; conversely, if it floats deeper then the stern of the new vessel will be a bit deeper when the extension is added.

 

Without any calculations, I have stretched a good many planing boats, and as long as the lines are followed accurately, the results have always been good.
It is a good idea to build cradles templated off the existing transom before cutting up the boat to retain the hulls shape, and insure that the extension remains the same.
It is a lot of work, so I would be inclined to go longer than the proposed 26”, as you can get more boat for nearly the same amount of materials and work.

 

Thank you for the advice kapnD I will definitely consider going longer, and make sure to retain hull shape with cradles as i build my melamine mold back.

bajansailor, this is what I initially thought, thank you for clarifying my thinking. I just wanted to make sure that it was actually that simple. Many times it seems like people go to make what seem like simple changes and then it ends up changing their vessel in a way they had not foreseen. I am an amateur at designing and modifying so i didn't want to assume something is simple if an expert could plainly see that it is not.

also the exercise of calculating water plane MT1 and CG has definitely given me a much better understanding of trim, even if it was over complicating things.

thank you guys for all your help

 

@dannyboy have you got the body lines plan of your boat?.
With it, it can be very simple and very fast to obtain a more reliable response.
Not only does the cog change, but so do the cob and float barycenter, all of which can lead to unexpected surprises.

 

I have looked but unfortunately i do not.

 

A freehand drawing with dimensions?

 

Here are a few drawings and a photo of the boat underway.
I posted the dimensions in my first post.

 

https://www.trojanboats.net/trojan-international-delta-conic-hull.html

this may also be helpful

 

Main issue with extensions...

Do not create a hook in the hull bottom.

Try to avoid a wiggle in the hull by breaking the hull at the lowest rate of change, or more simply a straighter section.

 

There is not enough data to draw your boat so I have opted to, based on what is on the web https://www.trojanboats.net/trojan-international-delta-conic-hull.html, draw a 20 foot delta-conic hull and then enlarge it 26 ". In the attached file I show you the results for various drafts, assuming that the boat floats with the keel level, that is, with zero trim.

 

wow thank you so much, this is the exact info i need. I take it the centroid of submerged body is COG and the centroid of flotation is COB?