Any math whizzes here? (C-Dory engine related :D)

[Sunbeam]

I have a question for someone who took more math than I ever did. It has to do with my primary engine - Yamaha F80 - mounting.

So, right now when the engine is tilted all the way up (boat at rest), the tip of the skeg just barely touches the water when the boat has a cruising load. That's the baseline.

I'm going to raise the engine to its highest mounting hole, which is 1-1/2" higher than the one its in now. This is for performance reasons because my Perma Trim is much lower in the water than it should be (Roy & Dixie have same engine/same mounting, did the raise about a year ago, and say it was a definite improvement.) Okay, so that part is clear. But here comes the math...

There is another issue I'd like to address, which is that I am constantly all the way to one end of my trim range when above hull speed. I have to always have the engine trimmed all the way down. (This is independent of whether or not the bow is down, btw.) I asked Roy & Dixie and they said that even with the engine raised, they still have that same issue. Anything but all the way down is less good. So... who knows if that is the best position or not, because there is no way to trim past that to where it gets worse going that way, then trim back to the ideal position. You can only trim all the way down at which point it's the best it can be, but you can't tell if it could be better because you can't go any farther.

So to potentially address that, I got a pair of 5º "tuck" wedges - they simply fit between teh engine bracket and the transom and make the transom "5º more slanted." Thus engine trims down a bit more, and up a bit less. If I'm going to install these, I'd like to do it when I'm moving the engine up (am also going to overdrill/fill/re-drill the engine mount transom holes, as these are the last ones I have not done). Likely these would give me an actual adjustment range, wherein I could trim "too far" in either direction, then fine tune it to just right, performance-wise. That would be nice.

However.... I can't figure out what this combination would do to my "skeg level" when the boat is at rest and the engine is tilted all the way up. On the one hand, the 5º "tuck" wedge would lessen the amount I could trim the engine up at rest; but on the other hand I'm moving it up on the bracket 1-1/2". I can see what either of these would do alone, but with them both done, where do I end up in comparison to where I am now for skeg height with "engine at rest and tilted all the way up"? I'm sure there is some math I could have learned in school to figure it out in advance - but I didn't.

BTW, the tilt degree on the Yamaha F80 is not shown in the specs, but the current 75/90 I believe have the same bracket/tilt mechanism and they show a 65º maximum tilt.

If math fails, I will try it with cardboard mockups

[rogerbum]

[BrentB]

What the OB length?

on the flip side

old joke said often at work

There was a statistician that drowned crossing a river... It was 3 feet deep on average.


Pizza


Write the expression for the volume of a thick crust pizza with height "a" and radius "z".

Source: Reddit

Explanation: The formula for volume is math symbol for pi times(radius)2 (squared) times (height).

In this case, pi·z·z·a.


I am accepting courtesy laughs

[potter water]

5 degrees is very tiny. So I predict that you will be very close to 1.5 inches higher with the skeg. With the many variations in loading, you will never really have a mathematically perfect prediction of where the skeg will be due to that tiny 5 degrees. Just be happy that it will be most of 1.5 inches higher and don't worry about the minimal change that the 5 degree extra down tilt will do. It sounds to me like you'd be as well off to work on re balancing the boat so you don't suffer with needing to lift the stern so much.

You'd get a lot more benefit from re-balancing your load a bit than from adding the wedge. AND you'd know that the skeg will be exactly 1.5 inches higher to boot.

You have so many vairables because of loading trip to trip that the math just isn't necessary.

Although, someone here will undoubtedly work it out just for fun. I'm in the ILAR school of mechanical design. (It looks about right).

[Sunbeam]

Good answers, everyone - thanks!

As far as re-balancing the boat, I agree that my boat is stern heavy. However, I think just about every 22 is (that has 23+ gallon fuel tanks and a modern 4-stroke engine). Note that Roy & Dixie have the exact same engine and same situation, and I think they may not even have a kicker (not positive though). At any rate I don't believe their boat is particularly heavy. I dislike weight in the ends as much as anyone.

From what I understand, the hull was designed for a 250# 2-stroke 70 and a couple of 18 gallon fuel tanks, plus maybe a starting battery.

I have eliminated the ~120# kicker in favor of a 45# version - that will remove some weight and should help with the starboard list. I may move my house battery forward (which would just leave a small starting battery). But other than that the only things I have aft of the cabin are the fuel tanks, a bilge pump, cleaning supplies and a few propane canisters in the port lazarette, the batteries and trim tab HPU in the stbd. lazarette, dock lines and fenders (2 only), , a reasonable sized cooler (48 quarts), a beach chair, a lightweight aluminum bimini frame, and misc. light items in the gunwale pockets. I can't see much there that is not on just about every 22 (?). I guess I'd be surprised if other cruising 22's are any lighter (and then would like to know their secrets!)

Do others tilt their engine up and have the skeg more than just clear the water when loaded for cruising?

I agree it is too much weight for the design, which I imagine is why the evolved hull (Cape Cruiser, Venture, Marinaut) has much more buoyancy aft. That said, it seems to suffer the added weight relatively gracefully, given what folks do with their 22's.

Back to the math problem: I see what you mean about the missing figures, and and "squishiness" of the problem, so I will go to the tape/cardboad/stick method, and/or just put the wedges on. Thanks!

Sunbeam

[potter water]

I don't think you said you had trim tabs in addition to the permatrims. My boat has the hydraulic tabs, and that just makes all the difference to the extent that I don't worry about loading. Between the engine trim and the tabs, I'm good for about any load. I'm guessing that the majority of 22's and 25's have hydraulic tabs or some kind of controllable trim tabs in addition to the engine(s). Mine came equipped that way from the prior owner, so I can't address the installed price to add them, but you'd like them. I'm sure they are a grand or two addition though.

You can always keep the hydraulic tab option on the back burner if you don't have them and if your current mod doesn't do the job. Keep us posted on the performance change when you get back on the water.

[Sunbeam]

I do have Bennett M-120 hydraulic trim tabs. I installed them last summer. I also installed a Permatrim. I can't tell exactly what the Permatrim is or isn't doing because there is no way to "turn it off," but I can definitely tell what the trim tabs are doing (since they can either be used or retracted). I would certainly miss them if I didn't have them.

I don't think there is anything particularly unusual about my boat. I say that because whenever I have mentioned the "engine likes to be trimmed down" or "Permatrim rides lower than optimum" issues I've had other 22 owners chime in to say their boat is the same. Lucky for me, Roy & Dixie (same engine/position as me originally) moved their engine up just as I propose to do, and they have since cruised thousands of miles. I PM'ed Roy yesterday, and he said no regrets and he found it a definite improvement. He said if he were me now, he would consider fitting the wedges at the same time as lifting the engine (because lifting the engine didn't impact the "trim down" feature).

I only thought to check to see if it would negatively impact my tilting of the motor at rest because Will-C had mentioned it in a different thread (pertaining to shopping for motors vs. tilt angles).

Since I don't moor my boat in the water, even if it does negatively impact ultimate tilt angle that won't be a big deal - I'd much rather be able to run as efficiently as possible. But I was still curious if it could be calculated, just because I like math/physics/etc. -- although don't have much formal training in them -- and it's just fun to figure out the answers to questions.

[gulfcoast john]

Sunbeam,
I can get the forward 'bulb' portion of the gearcase clear of the water if I trim full up, but if I flip down the brackets to retract the rams the 'bulb' will be 3/4" inch or so in the water. Slap a little bottompaint on there, it's no big deal. Dr Bob pointed out to me it's probably better to have the rams retracted/not rusting vs this tiny bit of lower unit in the water.
Sometimes I worry you're even more OC than me, and that would not be healthy.
Happy Boating!
John

[Sunbeam]

Hi John,

Thanks for your thoughts. When I lift the engine I can put the wedges in, or not put the wedges in. Figured I'd find out what they might "taketh away" in addition to what they will give me. Sometimes I find something out ahead of time that changes my plan (without actually having to "undo it" in real life.) The less undoing the better

[Chester]

One way to transfer weight foreword is to put the Porta Potty between the fuel tanks and slide the heavier cooler in it's place.
The anti ventilation plates on our motors are at the same height as the hull bottom which puts them under water while on plane. I raised the motors one hole to try to eke out a bit more efficiency. This caused ventilation and vibration issues so I put them back.

[drbridge]

During the time I have had our boat on the water, I too have wondered what would happen if I could get more downward tilt out of the permatrim on our honda 90. A wedge on the transom is one way to do it. I have considered making a wedge for the permatrim itself where it mounts to the motor. I could do this with wood for a prototype trial and if it worked well look in to having it made of aluminum.
Has anyone else ever heard of doing this, or any downsides of trying this experiment?

Doug

[Sunbeam]

Chester: I guess I'll have to see how my particular engine/boat does, but at least I have one example of the "raise" working out well (Roy & Dixie). And too, I could always re-lower it, like you did.

Doug: I hadn't thought of that. For some reason angling the Permatrim makes me nervous, but I can't exactly say why (?) (Maybe because I'd rather move the prop/thrust as well?) But also I just never thought of that (and I want to get the engine mount holes in the transom over-drilled and filled so this is a good chance to do it).

Just for the record, these are the wedges I now have on hand, made by TH Marine.



http://www.thmarine.com/products/Outboard-Performance/Outboard-Transom-Wedges/Transom-Wedge

I also found ones that are not cast (so maybe technically made of better material), but I didn't like those as much because since they are "universal" (i.e. the same wedges can shim the motor more down/in or more up/out) they are just slab shaped and not contoured to fit the bracket.



The TH Marine ones you have to choose which way you want to change the angle (up or down), because they are different products, but as a result they are more contoured to the bracket.

I think this is the same (or similar) wedge on the 22 Bixby's Cub (Texasair), although he has a different engine than I do.

[Chester]

Can we assume the pins which limit the downward trim are in their lowest hole?
I removed the wedges 'cause the pins were in the second hole from the bottom and I never used full down trim while under way. With the pins in the bottom hole the lowest trim is about the same as before. The motors clear the water when raised now.

[Sunbeam]

My engine has the holes, but on the models with power trim/tilt (which mine has), no pins are used in them. So the engine is free to be trimmed down/up as far as the trim/tilt mechanism will let it.

[Chester]

Hmmm... My 40's have trim/tilt.