Planing (again)!

A

aposner

New member
I've read every thing I can on C-Brats about trim tabs, motor trim and planing, but I'm still confused.

Upwind II is a CD-25 with twin Honda 90's.

My confusion is that I don't know when I on plane. My understanding is that as one increases speed the bow at first rises, than when planing the bow comes down again. My boat doesn't do that. I start with the motors trimmed all the way in, give it lots of throttle and get up to about 30 kts, but the bow doesn't seem to come down.

I add some down trim tabs, but that doesn't change anything.

At 18-24 kts I'm getting about 2 miles/gal on my flow meter, which seems low.

I don't really care about my speed. My concern is fuel economy and if I'm "plowing" through the water rather than planing, I'm certainly not being very economical.

I'd like to go faster than displacement speed i.e. (5-6 kts) but I don't want to be burning fuel in the zone between displacement speed and planing. It seems like that is the only zone I'm ever in.

I know some will say that if I reach 30 kts. I must be planing, but how can I tell?
 
where do you live? I would be more then happy to come run your boat and show you how its done. I live in western Washington.

In a 25 with twin 90s I would say that the best you are going to get is 2.3 to 2.5 mpg. a lot of that depends on load.
 
Hi you may want to start off with tabs retracted, engine mid trim ie neutral and power up. I only know the 22 so not sure how "bow high" you are. These boats typically "rise out of the water" not bow high like ski boats. In the 22 at mid load I figure I am about on plane at 11-12 knots. You feel a mush in between displacement and plane and the bow may rise a bit. Our planing angle is still never level. Not possible. You are planing way before 30 knots as by definition displacement speed is a function of waterline length. Generally accepted to be about 1.3 to 1.5 times the square root of your waterline length. Let's say it's 8 knots. Over that you start to plane. For several more knots you are on a mushy plane lets say to 14 knots. After that the boat is about as "up" as it's going to get. Many 25's will cruise slower than 15 for long periods.
Then trim for ride and economy. I believe a lot of 25 owners see 2-2.5 mpg in go fast mode. In displacement mode we all do better. My 2011 cd 22 has 195 hours and 1265 miles so shows my average just over 6 knots I yeehaw along at 12-15 for a while then slow down. I hope that helps. Watch a typical sport boat take off next time. Some get pretty bow high. It is not a good thing. Be glad your cd is properly designed. Best. George
 
Is the boat loaded too heavy in the stern? Are your expectations realistic? Do you need to sit on a pad to see over the bow?
Inquiring minds want to know!
 
Chester,

Yes the boat is loaded at the stern with 2 large motors, the water tank and fuel tank. But so are other 25's.

I don't have a problem seeing over the bow, but I'm tall.
 
You might search Perma Trim on this site. Might be the answer to get that bow down. Try adding some weight to the bow. See if that helps get the bow down. Have someone crawl into the berth area is an easy test.If you boats doing thirty you have to be on plane. You just have to use the trim tabs to get the bow down but not too much. Usually there is a happy spot to fuel economy at about 3500 to 4000 rpms. Going along at that speed adjust the motor trim and see if speed increases. You should be able to feel the difference in between too much bow up and to much bow down. Is the bottom of your boat clean?
D.D.
 
My dealer installed a transom motor mount wedge to provide a more bow-down cruising attitude. Seems like a good idea...

With just a little amount of experience on the boat, I can say that it does plane easily in an almost level attitude depending on trim settings. Caveat - this is with a single engine and low fuel, but full rear-mounted water tank.

Pete
 
Our 25 runs bow high, even when on plane. If you are doing more than14 knots or so, you are on plane... if not, the stern would squat even more and your speed wouldn't increase into the 20s. If I am doing 20 knots (with a single Honda 135), I am getting around 2.2 miles per gallon (according to the fuel flow meter and based on plenty of fill-ups with the boat). Slow down a bit, and the mileage will go up a bit. I find our best mpg when on plane happens around 16 - 17 knots with slightly better than 2.5 mpg. Slow down to displacement speed and you will see 5+ mpg.

With full down trim tabs, motor trimmed slightly up, our boat is reasonably level, but still bow high at speed. Those who have put on permatrims have reported better level/trim.

When we first got the boat, I thought it set bow high when running, but soon got comfortable with that. It was more than I expected before getting the boat.

A fuel flow meter will help you determine the best mpg at a particular speed, by adjusting the trim, but you have to be patient while it settles in. I have come to expect 2.5 mpg as a realistic fuel burn (with the single) when on plane. The boat doesn't "hop on plane and flatten out" the way a deeper v-hull generally does.

Sounds to me like yous is pretty typical. Hope that helps.

Best wishes,
Jim B.
 
Our boat is typically heavily loaded. We have twin Honda 40's, trim tabs and permatrims. We start with engines in neutral trim, trim tabs retracted. We increase throttle to 4200-4400 rpm. As we get up to 10-11kts, just before starting on plane, we trim the engines down. The bow drops noticeably. We typically do not trim all the way down. We now use the trim tabs for lateral trim only. The behavior you describe does not sound all that much off the norm. The C-Dory is not much of a "hole shot" boat. The permatrims were a good add-on. I'd suggest just experimenting a little more. Our boat on plane a high speed is still bow up some.
 
The C Dory 25 planing speed will depend on loading and trim. Generally on ours with a 130 and fully loaded for cruising the point of planing would be about 15 knots. A boat which is lighter will plane at a slightly slower speed. Between displacement speed (in the C D 25 this is going to be about 6.3 knots--and 13 to 14 knots will be semi displacement speeds and not as effecient as displacement or planing. After about 30 knots the boats become a bit squirrley and I avoid running faster than that speed (baslically due to flat bottom and lack of any keel). Your boat will probably run at faster than 30 knot speeds.

In your case, trimming the engine full down is probably more than is necessary for normal running. As I remember your boat also has trim tabs. I would put the trim tabs at about mid excursion, and the motor lower ujnit about 3/4 down to come up on a plane. If you want to come up faster, then full down on the tabs--but not the engines. You want to then retract the trim tabs for normal running fully. Set the throttle in one place, with both engines turning the same RPM and move the engine trim until the boat speed is at a max. That will be the most effecient running speed.

Don't worry about "bow rise" There will be some. If you push the bow down too much either with trim tabs or engine full trim down, then there will be the effect youj describe.

As for Mpg You should do better--about 2.2 to 2.5 with a light boat. Run with the water tank no more than half full to start with--assuming that the boat is not loaded heavily in the bow.

For our AK trip, we had, the boat loaded with all food and gear for a month--one ice chest, (about 200 quarts) and the Norcold freezer, a 1000 Honda generator, a 2.5 hp Suzuki outboard. We had extra canned food in a locker in the cockpit, but in our 25 the water tank was forward--holding tank aft. We averaged slightly over 3.2 mpg for the entire 1200 mile trip.On a plane we were about 2.5 mpg. But with less hp and only going 17 knots.

Good luck and continue to experiment with the new boat!
 
My boat is the same as yours: 25 Cruiser w/ twin Honda 90s.

What I've found after a lot of experimentation and cruising is that my economy (and ride) are best with the engines completely trimmed in. I know that does not seem to be the norm with how most people trim their engines, but maybe the added weight of the twin 90s makes us unique. I have no problem whatsoever getting the bow to come up to get on plane with the engines trimmed all the way in. In fact, if I don't trim them all the way in my bow can get uncomfortably high when coming up on plane.

Where we deviate is that I can definitely get the bow to settle down nicely once on plane....even without the use of tabs. I tend to put the tabs about half-way down though to smooth out the ride in chop as well as getting the boat to track better at max speed.

Other than displacement speeds, my best fuel economy is about 2.7nmpg, but conditions have to be perfect. Most times if I can get 2.0 or above I'm happy.
 
you will know from your wake if you are planing. fishtales /spray coming out from behind the engines.
I think Les advised me of this.

Oh and unlike a deep v boat the bow never comes down on a c-dory.(but it doesn't look as odd as a planing ranger tug). Because of the hull design there is no pronounced moment (like a deep v) when you know you are planing. You go from displacement, semi-planing to planing in a smooth transition. They just aren't meant to run that way. If you try running level (by using all means to drive down the bow) you will use more fuel and dig in at high speeds.

Don't fight it. You will get used to it. Just run it at the lowest rpms that you can with the bow as you'd like (I bet your optimal planing fuel economy will be around 16 knots)

Cheers
Stefan
 
Our experience matches Jim's. We do not have a fuel flow meter, however, so I cannot comment on GPH etc. We have a Perma-trim installed and it has allowed me to not rely in the trim tabs for bow adjustments, as I did before I installed it; we used to just trim all down in an effort to lower the bow. Of course this would negatively affect GPH. Trim tabs are now only for subtle lateral (port vs starboard) trimming.

The boat is "ass heavy"; 150 horse main, 9.9 kicker, water tank, fuel tank, cooler...all at the cockpit/stern. We put whatever can up front as much as possible and someday I'll put another water tank in the bow, but for now, we just do the best we can.
 
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