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Old 27-03-2017, 04:32   #1
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Dragging wet cement?

Saturday, we picked up our new tender. It's a 2008 10' Rigid Sport Tender. Center console, nav lights, VHF radio, and Mercury 15 hp four stroke outboard with electric start and alternator. We're very pleased with it except for one thing. When we push the throttle toward WOT, the bow comes up but it never pops up on plane and, in fact, seems to drag as if we were towing someone or if we were pulling 100 pounds of wet cement. Combined weight of the two of us is about 330 lbs. Since our old tender, an inflatable RIB with a two-stroke 6hp, would pop up on plane, we assumed a 15 hp four stroke would, too. Is this just how it is or should we have a respected outboard mechanic take a look?
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Old 27-03-2017, 06:20   #2
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Re: Dragging wet cement?

Lots to consider here. Unless the boat is a lot heavier than the old one, I would think that the doubling (+) of the horsepower would get it done. So, in know particular order:

1) Is it propped correctly?
2) Is the shaft length appropriate (the prop needs to bite "clean" water).
3) Have you tried "helping" the boat by temporarily moving someone forward in the boat while attempting to get up on plane? If so, what were the results?
4) In the alternative, can you trim the outboard?
5) Do you have a way of determining if the motor is making appropriate RPMs at WOT?
6) If the boat manufacturer is still in business, have you contacted them for advice? There should be a placard in the RIB with a horsepower limitation - if the 15 is close to the upper limit, that would likely be confirmation that 15 should do the job, but that is subjective opinion. See what the manufacturer has to say.

Any additional information (the more specific the better) may help you get better answers here. I would certainly think 15 HP would get the job done. What is the change (increase or decrease) in weight between the two boat/motor combinations?
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Old 27-03-2017, 07:21   #3
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Re: Dragging wet cement?

Quote:
Originally Posted by SFS View Post
1) Is it propped correctly?
2) Is the shaft length appropriate (the prop needs to bite "clean" water).
3) Have you tried "helping" the boat by temporarily moving someone forward in the boat while attempting to get up on plane? If so, what were the results?
4) In the alternative, can you trim the outboard?
5) Do you have a way of determining if the motor is making appropriate RPMs at WOT?
6) If the boat manufacturer is still in business, have you contacted them for advice? There should be a placard in the RIB with a horsepower limitation - if the 15 is close to the upper limit, that would likely be confirmation that 15 should do the job, but that is subjective opinion. See what the manufacturer has to say.

What is the change (increase or decrease) in weight between the two boat/motor combinations?
Some good questions, SFS. Should have included that info in the original post.

1) My knowledge of outboard propping is very limited so I don't know. We would have to get with an outboard mechanic to check into that.

2) It appears to be the correct length. When the tender is on the trailer and the motor is upright, the propeller is below the lowest point on the hull.

3) My wife did try to move forward when we were at WOT to try to pop us up on plane but it had no effect.

4) I believe there is a tilt adjustment rod. Has to be manually set before taking off. I'm pretty sure it's already set to the lowest hole, most upright position.

5) No tachometer on the tender. Would have to get with a mechanic to check the RPMS.

6) The manufacturer is still in business. I haven't contacted them yet but will do so today. Good suggestion.

7) Bonus question. The previous tender was an inflatable Avon. We believe it was around 180 pounds. Its out board was a 6hp Johnson two-stroke that weighed about 70 pounds. The new center console Rigid weighs about 300 pounds, according to the manufacturer's website, and the Mercury 15hp four-stroke should weigh about 125 pounds.

Does that help?
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Old 27-03-2017, 07:46   #4
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Re: Dragging wet cement?

the Johnson 6hp 2 stroke weighs more like 35lbs maybe as much as 40lbs. I have one.


The prop shouldn't just be below the lowest point, there is a plane on the lower unit. That plane should be just at the waterline when running at full speed, iow, that plane should be at the same depth as the bottom of your stern.


Does the engine sound like it's revving up really high when you WOT?
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Old 27-03-2017, 07:47   #5
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Re: Dragging wet cement?

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....Does that help?
It likely will, as other people chime in. I was asking more to start various thinking processes. I don't want to misrepresent myself as an outboard-powered dinghy expert, as I row my tender. It has simply been my experience on various forums that the more detail supplied when posing "troubleshooting" type questions, the more helpful the responses become.

For the weight of the vessel, 250 pounds has become 425. Total occupied weight has gone from 580 to 755. Not much of an increase compared the increase in HP, so thrust-to-weight ratio has only gotten significantly better.

I would call the boat manufacturer (or research the question online - I like talking to live folks, you can ask them open-ended questions). If they confirm that 15 HP is plenty, I think you need to look at whether or not the motor is actually developing rated HP, and whether or not that 15HP is being used effectively, which is a prop size/pitch/blade shape question, as well as one of placement of the prop in the water column. Prop questions can be researched online as well, and I would start on the Mercury site.

Other more knowledgeable folks will likely jump in over time, though I must confess, the title of the thread did not grab me as a "Dinghy Underpowered?" kind of thread. Don't know if a moderator would change it for you.

Edit to add a thought on boat trim: You said your wife "tried to move forward" to help the boat plane. The word try is a tough one to parse in print. Did the actually move a significant distance toward the bow, and it had to effect? Or did she attempt to move, and was unsuccessful in the attempt (perhaps due to sea conditions, incline of the boat, lack of handholds, slippery soles, etc?
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Old 27-03-2017, 07:51   #6
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Re: Dragging wet cement?

Okay, thanks. The clarifying questions are appreciated. I have written to the manufacturer and am waiting on a response.

Perhaps I should have given more thought to the title of the thread.
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Old 27-03-2017, 09:19   #7
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Re: Dragging wet cement?

Your dink is too heavy for 15hp.
15hp is good for a 10 foot dinghy at 200 lbs and 2 small to medium sized people, but you add another 100 lbs or so to the dink and you're over the limit.

My old dink, 9 foot Caribe, had 15hp and would plane fine with two, and even three people. But stick a fourth in there and you were plowing water all the way to your destination. Of course, it depends how big these folks are too. We could plane 4 of us little folks, but two 100kg + people, plus my wife and I at 60kg and 70kg meant a slow wet ride.

Put 20 or 25 hp on there and you'll be very pleased with it's performance.

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Old 27-03-2017, 09:27   #8
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Re: Dragging wet cement?

Rigid site says 300 pounds for the boat, and that the Sport 10 model will take up to a 30 HP motor. You may be indeed be underpowered.
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Old 27-03-2017, 09:44   #9
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Re: Dragging wet cement?

Quote:
Originally Posted by JohnEasley View Post
3) My wife did try to move forward when we were at WOT to try to pop us up on plane but it had no effect.

7) Bonus question. The previous tender was an inflatable Avon. We believe it was around 180 pounds. Its out board was a 6hp Johnson two-stroke that weighed about 70 pounds. The new center console Rigid weighs about 300 pounds, according to the manufacturer's website, and the Mercury 15hp four-stroke should weigh about 125 pounds.
Sometimes it takes a bit for forward weight to have some effect. Maybe sit her as far forward as possible, move yourself forwrd a bit if you can, crank it... and wait. Might get there, eventually. That kind of test would at least give you another data point for evalauation.


Quote:
Originally Posted by Scot McPherson View Post
the Johnson 6hp 2 stroke weighs more like 35lbs maybe as much as 40lbs. I have one.
Our '98 Johnson (Suzuki) 5-hp 2-stroke weighs 47-lbs.


Quote:
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Rigid site says 300 pounds for the boat, and that the Sport 10 model will take up to a 30 HP motor. You may be indeed be underpowered.
Our dinghy weighs 125 lbs, the 15-hp 4-stroke outboard is 107 lbs, the battery weighs another 10 lbs... and our dinghy will plane (relatively easily) with 3 pax, about 450 lbs of "pax payload." The minimum recommended HP is 10, the max is 15, according to the manufacturer's data plate on the transom.

Your dinghy and outboard combo is much heavier than ours. If they say it'll take up to 30-hp... they may well mean it won't plane with less than maybe 20 or even 25. Does the data plate list a minimum recommended hp?

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Old 27-03-2017, 09:51   #10
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Re: Dragging wet cement?

That sounds like a heavy dink. Not enough HP.
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Old 27-03-2017, 09:55   #11
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Re: Dragging wet cement?

I should add that you should try a certain hp before you buy. 20hp would be the minimum on that dink, and I don't know if it would be enough to get you and your crew/groceries, up on plane. 25 would be better.

Our new dinghy is a Caribe C12, it weighs about 200lbs. It planes quite well with 20hp, with four aboard. If we were bigger people, I'd put more hp, but as it is, it's hard enough to drag onto a beach. We have wheels on the stern for dragging the dink up a beach, and they're worth every cent.

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Old 27-03-2017, 09:58   #12
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Re: Dragging wet cement?

Does you dink have a drain plug for the hull? My Avon 310 hull will eventually fill with water and needs to be drained periodically. I'm guessing that can account for 30 or so pounds of water.

The Avon 310 with a 2-stroke Merc 15 (75lbs) and 305 lbs of humans can just barely get up on plane if all the conditions are just right- otherwise no plane.

So I suppose water in hull or not, your motor is too small.
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Old 27-03-2017, 10:11   #13
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Re: Dragging wet cement?

You may be pushing the bow down too much with the tilt rod in the lowest position. Try higher hole (more trim up) and go alone first. My old 150hp bass boat would plow the front with the trim all the way down. Would still plane but at a much reduced speed. If you can plane alone after getting trim optimal then try with the wife. If no go "loaded" drop the pitch on the prop.
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Old 29-03-2017, 06:47   #14
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Re: Dragging wet cement?

Quote:
Originally Posted by Tetepare View Post
Does you dink have a drain plug for the hull? My Avon 310 hull will eventually fill with water and needs to be drained periodically. I'm guessing that can account for 30 or so pounds of water.
Good point. Yes, we have a drain plug in the hull. When we remove it, there is only about a cup of water that comes out. We're suspecting that isn't the problem but we'll keep an eye on it.
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Old 29-03-2017, 06:48   #15
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Re: Dragging wet cement?

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Originally Posted by ctl411 View Post
You may be pushing the bow down too much with the tilt rod in the lowest position. Try higher hole (more trim up) and go alone first. My old 150hp bass boat would plow the front with the trim all the way down. Would still plane but at a much reduced speed. If you can plane alone after getting trim optimal then try with the wife. If no go "loaded" drop the pitch on the prop.
Good tip and suggestion. Thanks, ctl. We'll give that a try.
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