Wedge Between Motor And Transom - Why?

C

Chester

New member
There are aluminum wedges between the Honda 40s and the transom. They are approximately 1/2" thick at the top and 1/16" at the bottom.
The only purpose seems to be to move the lower unit back a bit while under way. Can this be right? What am I missing?
Do other C-Dory's have this wedge?
Seeing how the boat is a tad stern heavy moving the motor forward even a bit appeals to me.
 
Both C-Dorys I have purchased have had the wedges. My dealer says that the extra couple of degrees of engine tilt provided by the wedges gives a better ride in chop.
 
Chester":134lgm9v said:
There are aluminum wedges between the Honda 40s and the transom. They are approximately 1/2" thick at the top and 1/16" at the bottom.
The only purpose seems to be to move the lower unit back a bit while under way. Can this be right? What am I missing?
Do other C-Dory's have this wedge?
Seeing how the boat is a tad stern heavy moving the motor forward even a bit appeals to me.
Click to expand...

Chester-

If the wedges are as you describe,they function to allow the motors to tilt further up under the boat, which, when this happens, makes the motors' thrust lift the bow less, or even push the bow down, and in effect, to hold the bow down into the oncoming waves, allowing it to cut more into the waves rather than be lifted by them.

The geometry or mechanics of this dynamic thrust may not seem entirely clear to some, but is practiced by all when trimming the motors for the smoothest ride when in choppy waves.

The hulls of the C-Dorys are relatively flat, and will pound in waves of 1 to 2 feet or more at speed. The trick in trimming the engine to get the best ride under these circumstances is to trim the engines "down" (as described above, holding the sharper bow down into the waves and cutting them in half, rather than allowing them to slap against the flat bottom of the hull further aft.

The previous owner of your boat must have felt that the engines could now be trimmed down far enough to allow for enough down trim, so must have had the wedges installed.

Hope this makes sense as is not to repetitive, but I'm trying to make it clear!

Good Boating!

Joe. :teeth :thup
 
I noticed on my boat that the engine seems to "like" to be trimmed all the way down (i.e. lower leg as forward as possible) most (all) of the time. If there were a wedge as described, then there would presumably be adjustment either way from that position, vs. now where it is at one extreme end of the range.

(This may change on my engine when I raise it slightly on the transom, but if not, or if it's "normal" for the C-Dory, then it fits with the idea of a wedge giving better/more usable trim adjustment.)
 
There can be another trade-off with the wedges. Allowing the engine to tuck down further (thereby keeping the bow down) also means the engine will not tilt up as far. Depending on the tilt range of the motor, the mounting height of the motor, and the water line of the boat (especially when heavily loaded) the motor may not clear the water when tilted all the way up. This may not be a concern unless the boat is moored or anchored in the water a lot, and electrolosis, or marine growth fouling on the lower unit and prop are a problem.

With trim tabs, or Permatrims, and adjustments to mounting height the need or desirability of changing the transom angle may diminish or disappear. (Or even reverse, I suppose, i.e., at some point it might become beneficial to mount those wedges "upside down.")
 
I've never run with the trim all the way down as it slows the boat considerably.
Get this, the boat came with the pin that limits how far down the motor can trim in the second hole from the bottom.
 
I really think the necessity would depend on the particular boat configuration. My boat also likes to have its motors trimmed all the way in (down), even with trim tabs in the equation. A 25 with twins, 3 batteries, and 23 gal of water all at the stern (not to mention the 100gal of fuel) is a VERY stern-heavy boat.

My boat doesn't have the wedge mentioned in this thread, but wonder if it should. I'm fighting a constant battle with a stern and engines that are too low in the water while running.
 
I have never used transom wedges in any of the boats I have owned--including 3 conventional hull C Dories. It is rare that I trim the engine down all of the way. In some of the boats, I have found improved riding by adding the Permatrim, which seems to give more effect to the degree of trim. Only in the steepest of chop, have I wanted to trim the bow down. Usually by that time, I am running at a slower speed. Granted, the only twin engine C Dory I have owned is the Tom Cat 255.

If the boat requires wedges to run properly, I would consider that a negative or the two engines.

When cruising for a month at a time, our boats are heavy--with full tanks, extra 5 gallons of water, freezer, and ice chest in the cockpit.

The danger of running bow down too much is in down wind/down waves, there is the chance of both bow steering and broaching.

I have found, watching the fuel flow gauges, that the boats are most efficient in a relitatively neutral trim, to slightly bow down. Just my experience.
 
The good folks at Mobile East Marine are proponents of the transom wedge, so of course there's one installed on my 25 (with single Honda 150) which you can see in my album.

This just alters the trim range available to provide a slightly more bow-down tilt range. Even with the engine trimmed all the way down, at 15 knots and nearly full fuel the deck angle is still a bit bow up, and just about right in 1-2 ft waves. In flat water and 25 knots, level trim is good. In heavy weather, I still need to slow way down and trim bow up to avoid the aforementioned broaching problem.

The Bennett trim tabs do help raise the stern some, but it may be that installing a Permatrim would improve the trim authority and do the most good. It's on my mod list and feedback from those of you who have them on the 25 would be welcome.
 
PeterQ":q3sw902t said:
The good folks at Mobile East Marine are proponents of the transom wedge, so of course there's one installed on my 25 ...

This just alters the trim range available to provide a slightly more bow-down tilt range.
Click to expand...

That's what I was thinking. I have a 22, with Bennett trim tabs and a Permatrim. So far I have mostly only boated on Powell, but there my engine was always happiest trimmed all the way down (no matter what I did with tabs). Any deviation from all the way down just produced more noise and less speed, so I have to assume it was less efficient (for a given RPM). So, it seems that if I added a wedge, I would have more potentially useful adjustment on the "down" end, and would just give up a bit of never-used "up" end. There are two things I need to check though:

1) If I gave up some of the "up" end (with a wedge), would I still be able to trim the engine up high enough when not running (i.e. at rest).

2) Will things still be the same once I raise the whole engine slightly (which I plan to do as it sits a bit too low now). This will be done and things reassessed before I would add a wedge.

3) I may also jettison the heavy kicker that came with the boat.

Anyway, probably won't do any of this until I use the boat a bit more, but just thinking out loud. Granted the boat "shouldn't" need a wedge, but then (from what I know) it was designed around a 250# powerplant, 215# of fuel, and around 60# of battery - for around 525# total. I (and probably most others now) have 480# of powerplant, 276# of fuel, and around 140# of battery - 896# total. So maybe it's not too surprising that the original geometry needs a bit of tweaking. I imagine this is a good part of the reason behind the additional reserve buoyancy in the sterns of the newer designs (Cape Cruiser, Marinaut).
 
Edit: The wedges are 1" thick at the top, not 1/2" as stated in my first post.
Update: I removed the wedges and raised the motors a notch.
The good. Now the anti-ventilation plates aren't submerged while planing which my research suggests is desirable. The engines gained 100-150 rpm at full throttle and can turn 6,000 rpm with 13" props.
The bad. There seems to be a bit more structure borne noise while under way, the props blow out with a modest amount of upward trim and vibrate when trimmed down. This yields a rather small amount of useful trim.
By making two changes at once it is hard to diagnose these issues.
The motors came with the pin that limits how far down the motor can trim in the second hole from the bottom. I still can't fathom how the wedges help because by lowering the trim pins to the lowest position trim is similar with said wedges removed.
 
You are correct that the limit of the down trim may be adjusted, and if it was the same amount of trim down, is equal to the wedge, then why the wedge?

You want the anti ventilation plate just on the surface of the water (with the c Dory), not above it. This may be hard to achieve. I suspect that you are getting some ventilation on turns etc which is not desirable. I would drop the engine back down. I suspect that is the reason for the blow out, vibration etc….

I don't run with the bow trimmed all of the way down, except when trying to "cut" thru chop.. I try and trip for best RPM, at a given throttle setting. Of course the boats vary a lot because of the weight aboard, and the placement of this weight…

Thanks for the follow up post!
 
Chester,
I would guess that your boat was originally rigged by Sportcraft Marine in Portland, Or. My 2002 C-Dory 22 had the same wedges and it came from Sportcraft. You could argue whether or not they are necessary but from a practical standpoint the boat could still be trimmed in exactly the same manner with or without the wedges. The wedges would allow a little extra nose down trim from the engines, but when the motors are trimmed to a level position relative to the hull, it really makes no difference whether they are there or not.
 
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