At last a build thread: CR16 skiff

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Dan_Smullen
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Re: At last a build thread: CR16 skiff

Post by Dan_Smullen »

Awesome update, CR. Keep doing what your doin'!

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Re: At last a build thread: CR16 skiff

Post by cracked_ribs »

That's pretty much the plan - although maybe a minor detour into engine work will begin today, as all my parts and gaskets have arrived!

I suspect I'll be able to just about double the output of that old Johnson. Prop HP on it is around 17-18. I think I'll be over 30 when I'm done.

Of course there's about three props for these so I may not gain much. I have a 9.25x11 on there now. For sure I've seen a 12, but I don't think I've ever seen a 13 or more in that old 13 spline OMC pattern.
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Re: At last a build thread: CR16 skiff

Post by cape man »

When you dial it in, a good prop shop may be able to adjust whatever prop you have quite a bit if getting the right one with the 13 spline OMC pattern isn't possible. I had a stainless 13" pitch adjusted to an 11" when I first launched the OD18 and just did it because I was broke (half the price of a new prop).
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Re: At last a build thread: CR16 skiff

Post by cracked_ribs »

Yeah, it may end up that way - I had a really good prop guy where I used to live but so far, here, I don't have anyone (although I'm sure there are several shops that could handle it). I was thinking about it last night while heading to bed, though, and it shouldn't actually be too difficult to dial this one in. I haven't really gone through and checked these numbers but they should be close:

I can probably rev it to 5800, which I can verify with a cheap tach that's accurate enough for this kind of thing...gear case on these I think was a 2.15, so whatever that is, about 2700 turns at the shaft. 11/12 of 2700, minutes to hours over 5280 and something like 20% slip...that'd be around 22mph, just ballparking it. That's probably about what this thing is good for. So maybe an 11" wheel would be about right, anyway.

There's a dealership in the next town over that does OMC stuff; I'll have her out this weekend and see what I can do with the new bits on, then chat them up and see if I can try a couple of props. But I won't be surprised if an 11 is actually what I want, in the end. If the slip is too severe maybe I'll back it down - I never planned to run more than about 20 knots full tilt, if that. Probably more like upper teens.

In fact speaking of the new bits, the last of which arrived yesterday so of course after dinner I was out swapping them all in...

New intake is about double the volume of the old:

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Here you can see I've cut the gasket back to the size of the new intake, and then laid it on the old one so you can see just how much bigger it is all the way around:

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But here's the real money shot. The 20s didn't just come with a smaller intake and carb, they also had a restrictor plate to downtune them even further.

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I'm about to go from an 18mm throttle body diameter, to a 38mm diameter. What's that in area? ~250mm vs around 1100mm? The motor has enough compression you can hurt yourself trying to start it - in fact my neighbour did just that when we were goofing around with the original carb a couple of weeks back. But it's been breathing through a straw.

I'd say things are about to get real in Oyster Harbour.
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Re: At last a build thread: CR16 skiff

Post by TomW1 »

Hey CR give me the make, HP and year of your motor and I can look it up in my resources to get the gear ratio for you. That way you will know for sure if it is a 2.15 final drive. It's funny I grew up learning to drive a 25HP Johnson 14' aluminum V-hull boat. Take care and have a good summer.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

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Re: At last a build thread: CR16 skiff

Post by cracked_ribs »

Why thank you - it's a 1979 Johnson 20. The 20s are hard to look up and I find most people haven't heard of them, but the gearing should be the same as a 25.

I think they were all 2.15s at that point, and later ones definitely were...but off the top of my head I couldn't swear to it.
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Re: At last a build thread: CR16 skiff

Post by TomW1 »

cracked_ribs wrote: Wed Jul 14, 2021 12:19 am Why thank you - it's a 1979 Johnson 20. The 20s are hard to look up and I find most people haven't heard of them, but the gearing should be the same as a 25.

I think they were all 2.15s at that point, and later ones definitely were...but off the top of my head I couldn't swear to it.
CR I have spent the last 3 hours trying to find the Johnson 20 79 gear ratio. It appears not to be listed any where. The 25HP is 2.15 so I would go with that since the 20 is a down sized 25 and not an upsized 15. From what I found. I even found some for sale but they did not list the gear ratio.

Well again enjoy the summer.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

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Re: At last a build thread: CR16 skiff

Post by VT_Jeff »

My dumb idea: Remove the lower unit, spin the shaft and see how many turns it takes to spin the prop exactly 1 revolution. Maybe can do without removing lower unit.
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Re: At last a build thread: CR16 skiff

Post by cracked_ribs »

TomW1 wrote: Wed Jul 14, 2021 6:09 pm
cracked_ribs wrote: Wed Jul 14, 2021 12:19 am Why thank you - it's a 1979 Johnson 20. The 20s are hard to look up and I find most people haven't heard of them, but the gearing should be the same as a 25.

I think they were all 2.15s at that point, and later ones definitely were...but off the top of my head I couldn't swear to it.
CR I have spent the last 3 hours trying to find the Johnson 20 79 gear ratio. It appears not to be listed any where. The 25HP is 2.15 so I would go with that since the 20 is a down sized 25 and not an upsized 15. From what I found. I even found some for sale but they did not list the gear ratio.

Well again enjoy the summer.

Tom
Well I sure appreciate the effort on my behalf - I'm pretty confident they used the 2.15 as well but no doubt about it, the 20s are hard to get rock solid info on!

Jeff for sure if it come down to it I can crank it around - the flywheel is easy to access so I could always pull the prop, mark one spline and one tooth, and see just how close to 2.15 it is.

But I just can't see them using a different gearing when the only other change from the 25 was a restrictor plate in the carb. It'll be a 2.15 lower, I'm pretty sure of it.

I'll throw her back in the water this weekend and see what the new intake feels like - right now I'm just screwing around with the gutters on the forward hatch and maybe I'll get some of that framing glued up tonight. If I can get the hatch and seat sorted by Friday that'd be nice for any passengers daring enough to withstand the mighty power of the 22 cubic inch twin.
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Re: At last a build thread: CR16 skiff

Post by cracked_ribs »

A minor update since I'm kind of goofing off with the build right now...

Got the framing and gutters under the forward hatch sorted and just tabbed them in place by clamping them to the seat and putting a thick mix in at the hull, then went back afterwards and popped the seat off so I could do nice fillets around the surfaces I otherwise wouldn't have been able to see. This worked pretty well.

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Finally the purpose of the deck fill: it supports my grab rail, which is all the console I think this little skiff needs.

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I don't know why I did this this way, this was so dumb. I had previously cut out those hatches, and I thought I'd glass that whole bulkhead so I gently put the cutouts back in with a bit of masking tape as a spacer, then glassed both sides, and cut them back out. It worked, it was just a pointlessly hard approach to save about half a yard of fabric.

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Then I glued down the seat:

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And glassed it in, kicker still fits in and out nicely...

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And then I was like ah, screw building, let's just go get back on the water.

I think this is the only shot of the sheerline of the completed hull.

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The clipboard is for prop math (which of course being me I still just ballpark rather than take seriously, even when I bring a pencil and paper specifically to do the math.

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Every time I do this, I don't know why. You can see my liberal use of ~ which I use to mean "about." I'm pretty good at mental arithmetic but not very interested in actually doing math, so what I always end up doing is doing the math for a given step about 3/4 of the way until I think I have it within some reasonable margin, writing down the approximate value, and moving on. Partly I do this because I work with a bunch of engineers that are really persnickety and it drives them INSANE, although the thing is, I always calculate the values to within a useful degree of accuracy. They just hate that I casually dismiss the need to finish the equation. But I just keep an eye on my rounding and end up with values that are close enough and I find it really funny to treat math so roughly.

Oh, and I need to take a closer look at that tach. I hope I wasn't actually running at 6500! But it was sure howling at WOT.

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Anyway I didn't go back and look carefully so it's possible this napkin math done while running the boat isn't perfectly accurate. But clearly the prop that's on the boat is slipping like crazy. That's fine, I'll fix that. Actually I'll fab up a jack plate to tune the prop height, it's a tiny bit high with the amount of setback I have and between the height and the old behind-the-prop water pickup on this motor, at speed it starts to lose water. I didn't notice it at first because it's not obvious until you're really moving, but then I ran it for about five minutes at about 16 knots to get well ahead of a tug with a load of salvage logs and I realized it was getting way too hot! I shut it down for a bit, popped the kicker on just to be sure I could easily do so, and chugged around on my little 3hp Evinrude a bit before firing the Johnson back up and cruising slowly around.

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I brought some fishing gear along but once I realized I couldn't run at speed without drying out the motor, I just cruised around to look at some spots I'd been past, but never explored before. Once I get the bracket fabbed up it'll be more useable. Also, the beat up old prop probably isn't helping matters, it's churning the hell out of the water back there. I'll drop on a new one with less pitch and get the motor set back a little, dropped a tiny bit, and we should be in business.
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