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Old 07-28-2016, 09:47 AM   #21
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Dave,

How fast were you towing? Did the boat swamp? Did the dinghy yaw back and forth? (I always town with a double bridal with the bitter ends off both stern quarters to dampen the yaw.)

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Old 07-28-2016, 10:05 AM   #22
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dhays, same WB Genesis RIB but with a 15hp Honda. A miserable tow! In fact, a miserable drive at low speeds, even with the console. Nose high until it tips down on a plane.
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Old 07-28-2016, 11:05 AM   #23
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Fun with numbers:

I figure my 8' dink has about 40lbs of tension in the tow line at 7.5kts. Measured with my calibrated right arm.

7.5kts is about 750 ft/min or about 12.5 ft/sec.

40 lb at 12.5 ft/sec is 500 ft lb/sec.

One hp is 550 ft lb/sec, so my tow is taking 0.91hp.

My main engine is buring 1.9gph, and at this low power setting is probably about 15hp/gph, so about 28hp going down the shaft.

At that slow speed the prop slip is high. 12.02kt theoretical vs 7.5 actual. Applying this ratio to the boat power required, 7.5/12.02 x 28hp gives 17.5hp needed to push the main boat. The rest of the hp going down the shaft is wasted in prop slip. Sloppy math I know, but probably close.

So 0.91hp is for dink, 17.46 for total rig. So about 5% of my burn can be attributed to dink pulling.

Bigger effect could come from tweaking rpm by 25.

Now towing a big hardshell dink, those tow lines are TIGHT. It would be interesting for someone to put one of those 1000lb fish scales on a tow and see exactly how much pull is required.

Too hot to work and my tile guys did not show again. And I even got the AC going for them last night. Grrr. Don't ever build a house. Off to Lowes. Again.
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Old 07-28-2016, 11:12 AM   #24
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Towing Dinghys

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Originally Posted by DHeckrotte View Post
dhays, same WB Genesis RIB but with a 15hp Honda. A miserable tow! In fact, a miserable drive at low speeds, even with the console. Nose high until it tips down on a plane.

The dinghy and motor combo came with my boat when I bought it. I vowed not to do anything about them until I had lived with them for at least a year. They were bought by the PO when he first bought this boat and I don't think he had much experience. The RIB is a good boat. The Honda 8hp 4-stroke is a great motor, and with power tilt and electric start is much nicer than any outboard I have owned previously. The problem is that they are a terrible combination. The 8hp doesn't have enough power to get the RIB up on a plane. I have the version with a floor but no console, so heavier than some RIBs but lighter than yours. While not being able to plane is fine, the motor is so heavy that, as you said, it is always bow high.

I have considered selling the motor and putting my Torqueedo outboard on. It won't plane the RIB but is a lot lighter, cheaper to maintain, and doesn't require gasoline. I would go any slower than I do now and would do it more quietly and with less wake. It also might tow better since I could easily take the motor off and apart. My wife would like a RIB with a console that will plane, but my crane is only rated for 600lbs so not sure I want to spend the money.
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Old 07-28-2016, 11:13 AM   #25
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Too hot to work and my tile guys did not show again. And I even got the AC going for them last night. Grrr. Don't ever build a house. Off to Lowes. Again.
That's where I'm lucky. Wouldn't know how to build a house, so only way I could do it would be a contractor. So far we've never gone that route, although we have done some significant remodeling of houses we purchased. Always contracted.
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Old 07-28-2016, 11:20 AM   #26
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Dave, I don't think your issue is totally related to weight on the stern. I think it is a function of speed. You are right at the speed where the dinghy wants to plane. If you could go 11 knots or more, your dinghy would probably pop right out of the water and happily glide along on plane.

With that said, going from an 8hp to a 25hp would not increase your weight greatly....at least I don't think. Especially if you could find a used two stroke...which is likely similar in weight to your 8hp four stroke.
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Old 07-28-2016, 11:23 AM   #27
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dhays- Can you rig a piece of split pvc pipe and hose clamp to make a tiller extension? If you can sit a few feet fwd that dink might plane.

For a while I had a 4hp evinrude two stroke on my dink and it would only plane if I rigged an extension. Even with the extension, a small cooler with six beers, a gallon of bilge water, a full meal in my belly... Any of those would prevent planing, it was that marginal.

BnB... One and done for me and this house crap. About 90% done at this point.
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Old 07-28-2016, 11:37 AM   #28
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Last summer we towed our 15' Klamath aluminum fishing boat from the Seattle area to the Broughtons and back a little over 800nm with no discernible change in fuel consumption. It towed like a dream even in 5-7' seas we encountered on the way from Comox to Nanaimo.
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Old 07-28-2016, 11:41 AM   #29
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"I have considered selling the motor and putting my Torqueedo outboard on. It won't plane the RIB but is a lot lighter, cheaper to maintain, and doesn't require gasoline. I would go any slower than I do now and would do it more quietly and with less wake. It also might tow better since I could easily take the motor off and apart. My wife would like a RIB with a console that will plane, but my crane is only rated for 600lbs so not sure I want to spend the money."

Dave:

I bought my present dinghy here on TF from a guy who was downsizing to one like yours, for the simple reason that his (owned) slip wouldn't accommodate his newer, bigger boat with the Caribe 12 on the transom mount he had used previously, and was too heavy for his 600# capacity crane. I would be interested to hear his comparison between the two dinghies after a few years of using the WB.

My swap was a significant upsizing, from a 10' RIB with a 20hp 2 cycle, to this 12' with 40hp and a console. It tows well, cruises very well at 20 knots, and I can lift it onto my davits. If I was in your present shoes, I would listen to your wife, and do whatever that requires, to get a faster, planing dinghy with a console. It is for her after all.
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Old 07-28-2016, 11:46 AM   #30
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Quote:
Originally Posted by Ski in NC View Post
dhays- Can you rig a piece of split pvc pipe and hose clamp to make a tiller extension? If you can sit a few feet fwd that dink might plane.

For a while I had a 4hp evinrude two stroke on my dink and it would only plane if I rigged an extension.

Tried using an extension and still it would not quite plane. Likely I am just too fat. I will try again.
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Old 07-28-2016, 11:48 AM   #31
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Dave, I don't think your issue is totally related to weight on the stern. I think it is a function of speed. You are right at the speed where the dinghy wants to plane. If you could go 11 knots or more, your dinghy would probably pop right out of the water and happily glide along on plane.

With that said, going from an 8hp to a 25hp would not increase your weight greatly....at least I don't think. Especially if you could find a used two stroke...which is likely similar in weight to your 8hp four stroke.

You could be right about the speed. I never thought of that.
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Old 07-28-2016, 11:57 AM   #32
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My swap was a significant upsizing, from a 10' RIB with a 20hp 2 cycle, to this 12' with 40hp and a console. It tows well, cruises very well at 20 knots, and I can lift it onto my davits. If I was in your present shoes, I would listen to your wife, and do whatever that requires, to get a faster, planing dinghy with a console. It is for her after all.

That is good advice, "happy wife, happy life" and all...

Two things that I really like about this boat are the Sea Wise davits and the Stealhead Marine crane. The option to either have the dink on the boat deck or on the davits is very useful. I wish I had the #800 version of the crane, but then I am not sure I would want that much weight up that high. Maybe being limited by the #600 capacity on the. crane is a good thing.

I am sure that all it would take is $$$. I could find a dinghy/motor combo that would have a console, be able to plane with 2 people (even big people like my wife and I), and be light enough for my crane and davits. Unfortunately, the $$$ is an inconvenient reality at this point.
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Old 07-28-2016, 12:08 PM   #33
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I have a West Marine single floor rib with an 8hp, 4 stroke Yamaha on it. It is pull start so it is lighter by a few pounds.

It flies with me and still gets easily on plane with a second 120 lb person onboard and I weigh around 220.

It planes well with me sitting right behind the seat with a 18 inch tiller extension and the other person on the seat or on the side ahead of it. Weird...but it is what it is.

Might try fiddling with engine trim and sitting wayyyyy forward, you might be suprised....and I hope you are not insulted if you have tried it all.

Either that..... or a different prop... or Honda needs a new hp ruler.
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Old 07-28-2016, 12:23 PM   #34
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I will try some things out psneed. And no, I am not insulted by suggestions.
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Old 07-28-2016, 12:27 PM   #35
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We have the same WB 10 with a console and 20hp Honda, it won't plane with 2 people sitting side by side behind the console... With one forward and one driving it will lift the stern out of the water.... Boat probably needs a 30hp to perform or I could lose some weight... This combo doesn't work for me which is why we tow a larger boat when we go for a longer trip... By the way have you tried towing the inflatable with a bridle that has two loops, the loops go around the tubes at the rear and rest against the transom... the middle of the bridle goes through the front eye forming the attachment point for the tow line... This way the load of towing is not the forward eye but the where the tubes attach to the transom...
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Old 07-28-2016, 12:42 PM   #36
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By the way have you tried towing the inflatable with a bridle that has two loops, the loops go around the tubes at the rear and rest against the transom... the middle of the bridle goes through the front eye forming the attachment point for the tow line... This way the load of towing is not the forward eye but the where the tubes attach to the transom...

I am trying to picture this... You create two loops that go around and behind each of the tubes but as they go forward are run through the towing eye on the bow? The back end of the loops apply the force on the back of the transom but are guided by the towing eye?
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Old 07-28-2016, 12:52 PM   #37
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The written description is tough to visualize. if you look from above the bridle loops will go around the tubes at the stern. the bridal lines will go forward on the outside of the tubes then pull through the forward eye... most inflatables have a ring,handle something on the side of the tubes you can run the bridal through...
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Old 07-28-2016, 01:02 PM   #38
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The better it rowed should mean the less power it needed to be towed.

Actually when you are rowing a dinghy you are at displacement speed, and it does not take much power to move the boat. Towing a rowing dinghy at less than its hull sped requires little power. The best hull form for rowing displacement speed is not the best hull form for planing speed. This is why "rowing/towing" dinghies tend to have chines such as you see on a Dyer an its imitators. As others have said, using my calibrated armstrong tensionometer there is a tremendous load when towing a nice rowing dinghy above its hull speed. A planing dinghy is much less load at planing speed.
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Old 07-28-2016, 01:09 PM   #39
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The better it rowed should mean the less power it needed to be towed.



Actually when you are rowing a dinghy you are at displacement speed, and it does not take much power to move the boat. Towing a rowing dinghy at less than its hull sped requires little power.

This simple concept came back to me again yesterday as I spent a couple hours kayaking. The kayaks are 12 feet. They are very easy to paddle at a certain speed. However if you try increase the speed, you get to a point where the effort increases exponentially. The same holds true for any non planing hull, but to feel it in my muscles made it more "real".
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Old 07-28-2016, 01:11 PM   #40
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Another thing as it relates to planing your dinghy.....do most of you use those wings bolted onto the base of your outboard???? They do work....in fact they are a must if you expect your dinghy to plane. Without it, regardless of how much power, your dinghy will just point at the sky when powered up with only one person. With it, it will just pop out of the water almost level. Anyway, I don't mean to make this about dinghy planing and not dinghy towing.

I also tow my dinghy with the two side rings and not the center eye. I have a West Marine Dinghy. I use a West Marine towing bridle and tow the dinghy as instructed by West Marine. It actually works!!!
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