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Help with 18' Cat Craft catamaran

Old 04-10-2021, 11:21 AM
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You are fighting two different things on your current boat setup. Oddly enough they need completely different answers. I think simplifying the setup is going to help though. That crazy far setback is most likely helping your ventilation, not hurting it. Mine got better when I added the jack plate and multiple owners have had good results with the next size up motors from what you and I have. 150 merc, yamaha, etc. But that big set back is is most definitely hurting you from getting your bow down.
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Old 04-10-2021, 11:22 AM
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Originally Posted by Friends call me Whiskers View Post
Hey OP. How do you know that 1.5” up is the best cruise performance? Do you have fuel burn number readings? What are you basing that off of? Reason I ask is my Caracal runs most efficiently with the jack plate buried. If anything I could drop my motor down a notch to see if it gets even better. And it appears that mine is mounted lower than yours. I will try to upload a pic of it all the way down.
I don't have any fuel burn numbers, that's just the height that gets me the fastest speed at a given throttle position. I suppose that's not the same thing as most efficient though.
Old 04-10-2021, 11:29 AM
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Originally Posted by Friends call me Whiskers View Post
You are fighting two different things on your current boat setup. Oddly enough they need completely different answers. I think simplifying the setup is going to help though. That crazy far setback is most likely helping your ventilation, not hurting it. Mine got better when I added the jack plate and multiple owners have had good results with the next size up motors from what you and I have. 150 merc, yamaha, etc. But that big set back is is most definitely hurting you from getting your bow down.
Interesting, that's for the input. Visually it looks like we have the same jack plate and it doesn't look like your cavitation plate is much if any lower than mine. Maybe it'll be worth removing the spacer first to see what that does. One thing I noticed is that my engine is mounted pretty high on the jack plate, but I don't think there are mounting holes available to drop it any lower.
Old 04-10-2021, 08:40 PM
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Keep in mind, this boat will always have a ventilation sound. Nature of the beast of a single engine cat. If you get a lot of weight off the stern and get that bow down, I think you will be happy. My boat runs most efficiently around 4700 rpms and as much trim as you can give it before porpoising. Trim all the way down to launch and jack plate down too to get to cleanest water. You are going to want to get that bow up as high as possible before porpoising in calm waters. In the snotty stuff, you will have to get the bow down to cut through and avoid bouncing and bottoming out the tunnel. Just beware of some bow steer when you do that. How much battery weight do you have in it and where is it located?
Old 04-11-2021, 04:36 AM
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Originally Posted by Friends call me Whiskers View Post
Keep in mind, this boat will always have a ventilation sound. Nature of the beast of a single engine cat. If you get a lot of weight off the stern and get that bow down, I think you will be happy. My boat runs most efficiently around 4700 rpms and as much trim as you can give it before porpoising. Trim all the way down to launch and jack plate down too to get to cleanest water. You are going to want to get that bow up as high as possible before porpoising in calm waters. In the snotty stuff, you will have to get the bow down to cut through and avoid bouncing and bottoming out the tunnel. Just beware of some bow steer when you do that. How much battery weight do you have in it and where is it located?
Right now I can barely stay on plane below 5000 rpms, but it will be interesting to see what this work changes in that respect. I might try to modify the jack plate to allow for a lower mounting position. Right now most of the "up" travel is unusably high.

I have three batteries. One in the starboard stem hatch for cranking and two in the center console for the now removed trolling motor trim tabs. I'd like to get a bow mount trolling motor in which case I'd have to keep at least one if not two of those batteries. My boat also has a t-top with electronics box which I'm sure adds a good amount of weight.
Old 04-11-2021, 05:54 AM
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I have a 140 zuke and I’m pretty sure I plane at or just after 4000rpm.. I have a trolli up front. I would recommend a 60” shaft. I have the motorguide xi5 and us it all the time. I would also recommend a 24V system. I have two 125ah batteries and have never run out of power (I have an alternator charger that helps too though). I will be switching all 4 of my batteries to lithium to save about 125lbs (I have a deal I can get through work). As mentioned multiple times before, these boats are really weight sensitive, and I will probably get 3-5% faster at same fuel burn. The weight can make that big of a difference. The fuel efficiency doesn’t really fall off on my boat until over 5000rpm. This boat likes to run on top.
Old 04-11-2021, 06:41 PM
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Today I removed the spacer and jack plate and mounted the engine directly to the transom. I was planning to try just the jack plate first, but I discovered that it doesn't fit between the gunnels/splash guards on the transom. That probably explains why the previous owner added the extension. In the future, I may trim the splash guards, but I'd like to avoid that for now. I mounted the engine on the second-lowest hole (I'll post pictures later), which is at least an inch lower than the previously lowest jack plate setting. If nothing else, taking off the jack plate and trim tabs removed about 100 lbs from the stern of the boat, which should only help. I'm hoping to put the boat in the water later this week and will report back on how it performs.
Old 04-12-2021, 06:44 PM
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Here's the current mount height.

Old 04-12-2021, 07:54 PM
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Looks pretty good to me.

20" motor??
Old 04-13-2021, 03:53 PM
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Originally Posted by Bullshipper View Post
Looks pretty good to me.

20" motor??
Correct. 20".
Old 04-15-2021, 06:33 PM
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I was able to take the boat out for a brief test ride yesterday evening. I still have some ventilation while accelerating to plane, but overall I would say that my situation is greatly improved after taking off the trim tabs, jackplate and spacer. My engine now tops out at ~6000 RPM at full throttle and I can stay on plane down to about 4300 RPM. I didn't notice any ventilation while cruising and the boat seems to handle pretty well, although does have some squirrely behavior trimmed up at full throttle. I forgot to bring my GPS so I can't comment at this point about how/if my top speed changed, but next time I take it out I'll check. I'm looking forward to giving it more of a workout in choppy water, but so far I think these changes brought the boat to where it needs to be. I'll probably tweak it a bit more in the future, but it's good enough to fish out of now. Thanks again for all of the advice and recommendations.
Old 04-15-2021, 10:59 PM
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TRIM
Good going, sounds better, but it still sounds like you are running a bow light if I understand you right.

If the motor is reaching 6000 rpms, not overly trimmed up, and you need 4300 just to stay on plane it can be that you are still overly trimmed up, or if the trim is ok, lack a lot more more pitch on your prop.If it is bow light it will not run near as well, riding on top in chop. Being able to feather the bow is a big deal in a small cat in head and following seas.

Ideally you should not require more than 2500-3100 rpms on get on plane unless you are overloaded. If that is impossible or its still bow light after moving weight, then you will have to add some 5 degree motor mounting wedges from Bob's Machine shop for around $50 to get the motor to tuck in at a greater angle. The Permatrim, and the wedges will get the bow down as much as you want and I had to go this route on one of my cats as another 300 lbs on the bow would get that bow down. And with the wedges I could bury the bow w/o the extra weight giving max trim control.

PROPS
Once the trim is corrected your boat will be faster and you will need less speed and rpms to get on plane. Ideally, planning will happen around 13 mph at 2500 rpms, not 4300, if you have the right trim and props. More pitch and cup can be added to your prop to get more speed with less rpms, or you can borrow a used prop from Prop Gods after correcting the trim to dial in exactly what you need..

So, its up you you if you want to keep going, and if the trim is still a problem $50 is not going to break the bank to make another big step forward.

Last edited by Bullshipper; 04-15-2021 at 11:11 PM.
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Old 04-16-2021, 03:47 AM
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Thanks for the info. I actually have another prop to try that is the same pitch, but has significantly more cupping than the one on the boat so I'll try that next. I'll also look into the wedges you recommended.
Old 04-16-2021, 09:13 AM
  #34  
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Testing in seas, then the other prop are good ideas before you spend money and any more time.
I get where you are coming from when you say that's enough for now.
I'm just trying to provide more info if and when you feel like it.

Small cats are not easy to get right, and single engine setups are even more of a challenge.
But if you can feather trim your bow pressure going into seas, then you can goose it to get a much better ride and then your small cat will be so much smoother and dryer than any mono of equal size.
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Old 04-17-2021, 12:55 PM
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We sold an 18' Leader Cat and it never liked to be trimmed. Once you start to trim it up, it starts cavitation. The engine likes being deeper with all the dirty water created from the tunnel. Leader Cat reached 40mph with a 115 Yamaha and 3 blade prop.
Old 05-04-2021, 04:23 PM
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I got to test the boat out this weekend in some chop and with a few friends onboard. I'm still having difficulties getting the bow down and I'm getting some ventilation during cruise. I think I'm going to try lowering that engine to the lowest mounting hole and/or some of the mounting wedges next.
Old 05-04-2021, 04:56 PM
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Originally Posted by Shooterschmidty View Post
I got to test the boat out this weekend in some chop and with a few friends onboard. I'm still having difficulties getting the bow down and I'm getting some ventilation during cruise. I think I'm going to try lowering that engine to the lowest mounting hole and/or some of the mounting wedges next.
You need to do both.

Lowering the engine will help with the ventilation, but not with getting the bow down.

The 5 degree wedges will lower the bow under power.

Old 05-05-2021, 07:10 AM
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OP, I got your PM about mounting holes. I figured I would post it here in case people in the future have similar issues. So my engine is very likely mounted higher than yours. The shop I had the Atlas jack plate put on raised the outboard at least two inches from where it was originally. Usually, you raise the motor up 2 or more inches on a 6 inch setback. So yeah, it would help with ventilation issues if you if you drop it to the bottom hole, but this also makes me realize that we aren’t comparing apples to apples. A caracal would have zero issues jumping on plane with a 115 when outboard is mounted on the second to last hole.
Old 05-05-2021, 07:12 AM
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I wonder how different the construction is on your boat compared to mine? Is the 115 running OK? Can you get the boat over 30 mph? Are there any super heavy accessories that we don’t know about?
Old 05-05-2021, 05:31 PM
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Originally Posted by Friends call me Whiskers View Post
I wonder how different the construction is on your boat compared to mine? Is the 115 running OK? Can you get the boat over 30 mph? Are there any super heavy accessories that we don’t know about?
Thanks for the response. The 115 seems to be running fine so I don't think that's the problem. I tested max speed on a glass calm lake back when it still had the jackplate, trim tabs, etc still instilled and I was able to hit 33/34 mph.

​​​​​​The boat does have a fairly large T-top with electronics box and a couple of LED lights, but that's it for accessories. I'm starting to wonder if perhaps my hull is waterlogged. I didn't think that it was since I was able to go over 30 mph, but extra weight could be the cause of my issues. My bilges do have plugs, but I'm honestly not sure if those are connected to the rest of the sponsons or not. I don't see an obvious connection, but it's hard to see so I need to pull out my inspection camera.

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