Which Permatrim?

The best thing to do would be to talk to Andy Jr. there at Shipyard Island Marine. He's sold most all of us a Permatrim for our C-Dory's and is very familiar with the right Permatrim to choose from.
 
You may want to run the boat for awhile to decide if you want the Permatrims. I don't think that they are as crutial to performance as with the C Dory 22 or 25. What I find with mine, is that my planing speed is lowered--maybe to 11 to 12 knots--some folks say 9 knots. At 9 knots, I am throwing a lot of water up over the outboard bracket. What they do do is allow you to plane at a lower speed. However. I have found that in rough water, I want to go as fast as possible to have as much air cushion as possible--up to a point. For me that point of comfort is a bit over ture 3 foot chop. (that is pretty healthy chop). If more than that--maybe a little, than I drop the speed down--but eventually there comes a point of comfort--that you need to drop off a plane, and buch into it--and the permatrims don't do anything. The boats get up on a plane just as fast with the permatrims. You don't need to trim the bow down, as you do in a C Dory 25 in chop. In fact if you put the bow down too much in the cats, it can lead to handling problems.
 
thataway":dr8n7asb said:
You may want to run the boat for awhile to decide if you want the Permatrims. I don't think that they are as crutial to performance as with the C Dory 22 or 25.
Click to expand...
I hear you. But I wanted to add the primarily becuase of the lower planning speeds, which I equated to more efficient fuel consumption. And with gas prices on the rise, it seem a good idea. Am I off base on that?
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Mike
 
I don't think so Mike. I've got them on my TC with Suzi 115's and they made a lot of difference in being able to get up on plane quickly and stay there at lower speeds.

Charlie
 
I used the M8 on mine - and it doesn't fit without modification. I've only used it twice so not much experience. I can trim the boat a LOT better - finally get the nose down ! It also adds a little to the slow speed helm response (probably high speed also but i didn't notice it as much). Now whether the added response is good or not, it takes some getting used to cause you can kick the stern around pretty quickly, the boat will then run sideways for a short distance before the turn is actually started - is this AT ALL understandable ?
 
Mike, with my flow meters, the mpg is actually less at the very low planing speeds--you force the stern up and bow down and that uses more hp. The most effecient speeds are from the 18 to 25 knots--and close to level consumption at those speeds, so we usually cruise in the 22 to 25 mph range.
 
I confused. It sounds like the permatrims allow the TomCat to get up on plane, and stay there, at lower speeds. So how does that not save fuel?

Sorry, I must be missing something.
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mike
 
The speed between displacement speed and planing speed is sort of a no man's land for a planing cat (Tom Cat). The Tom Cat is not effecient at the semi displacement speeds which are in the 7 to 16 mph range. For example the Glacier Bay is a semi displacement boat, and gets better fuel mileage at the lower speeds, even though it is not on a plane.
At about 18 to 20 knots the C Dory begins to get better fuel effeciency. See links below. The Tom Cat has flat aft sections of its hulls, the displacement cats have narrower and rounder aft full sections although they cut thru the water more efficiently, they will not plane and the top speeds are less.

The permatrim is artifically forcing the bow down, and the stern up. This takes extra hp and fuel-- as the motors thrust is displaced more up aft, than forward--than the boat's natural planing speed. (This also happens with the conventional hull and low planing speeds, bow down, in the C Dory 25).

Just because the boat is on a plane, at a low speed, does not mean better fuel economy. We put the permatrims on to get a lower planing speed--and give a better ride--than to save fuel. Overall, I suspect that my fuel mileage is slightly less with the Permatrims than without them. But I can tell you it is definately more at 12 knots planing than at 18 knots plaining--and that is less effecient. .

Example: for both engines on my flow meters I will run about 10 gallons an hour for 22 mph. This works out to 2.2 miles a gallon. Lets say that I use 8 gallons an hour at 12 knots, this is only 1.5 miles a gallon! Somewhere in my past posts, I actually gave the numbers for those lower speeds. On the other hand if I was trying to run at 12 mph without the permatrims, I may be using 12 gallons an hour--and only 1 mpg.

Does that make it clearer? My apologies, if I assume things, I have been fooling around with this type of thing for over 60 years and tend to assume that folks understand some of hull dynamics. A good book to own is "The Nature of Boats", by David Gerr. Not much about cats. Malcolm Tenant had some very good explainations on his web site, but he was killed in an auto accident last year unfortunately. Some of his articles and graphs survive on the net:


http://www.2hulls.com/archive/Gen%20Art ... aning.html


http://www.catamarans.com/news/2006/04/ ... arison.asp

Read the text and pay attention to the graphs of displacement vs the planing hulls (both cats and monohulls).
 
OK I think I got it. Permatrims lower the planing speed, but are likely less fuel efficient.

Let me tell you why I was asking about this:

When I did the TomCat sea trial on the Chesapeake, we were in 3ft chop, and I really liked its stability, especially in beam seas. Now I know that 3ft chop is nothing to the seasoned Mariners here, but my boating experience before now is largely smaller deck boats and toons, where 3ft chop was a big deal.

Another thing that I really like about the TomCat was when it was time to call it a day because a surprise storm blew up, it had no trouble staying ahead of the storm, literally flying back to the marina.

But the thing is that I actually prefer to not to be in a hurry on the water. I am really not into the speedboat thing. There is too much beauty that needs be enjoyed while lazily motoring about. But, I knew from the get-go that the TomCat was more fuel efficient on plane.

So, now you can see why I was wanting to lower the planing speed. Be as fuel efficient as possible, but still be able to take my time and enjoy God's beautiful blue.

Know what I mean?
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mike
 
I know my post on this subject is years later than the last, but thanks to reading everyone's comments, I finally added Permatrims to my TomCat. While I only ran ViewFinder for a few short sprints up and down the river near home, I am very impressed with how she feels and handles. Before Permatrims I had trouble keeping on plane below 16 knots. Now the bow stays down and I can stay on plane at 14 knots. I'm pretty sure that after I have a chance to play around with trim in different conditions, I can plane at even slower speeds. With a very nasty chop at the mouth of the river, I found her much more comfortable at 16 knots than before because of a better angle to the seas. (Bow was down a bit more than before). WOT still got me 38 knots as before. So....so far so good....and I'm sure once I get used to trimming with these, I'll be even happier. Well worth the modest investment.
 
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