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Old 03 February 2008, 06:36   #1
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Country: USA
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Zodiac suddenly slows before planing

Hi. This is a 4 meter soft bottom Zodiac (aluminum/hard deck) with a 15hp 2-stroke and when I give it full throttle and as it begins to plane it immediately slows down with a big tug, big time. It almost feels like the boat is being pulled under as the nose is still pointed upright to a certain extent. After a few seconds it smooths out and gets on plane. The inflatable keel was solid. The confusing part is that it doesn't happen all the time. I did have some gear at the bow, could this be a factor?
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Old 03 February 2008, 07:11   #2
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Hi
I have a 4 meter Avon sib with a 15 hp 4 stroke. I think several variables have to be considered regarding your concern, including center of gravity,total load, and the angle of your engine. Probably the rate of application of power to full throttle is also significant.
Weight forward, unless extreme, should help you get on plane quicker.
With my boat and a heavier motor I have never noted such dramatic initial drag as you have described. My engine is about 110 pounds and I weigh about 180.
I recommend you check out the zodiac rib trim tabs on ebay because when I installed them on my boat it popped up on plane very quickly at a much lower power application. The greatest benefit is that when I decelerate water no longer pours in over the transom.
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Old 03 February 2008, 07:23   #3
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Weight at the front helps a lot - in fact I usually get someone to lean right over the bow and then move back when on the plane.
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Old 03 February 2008, 09:31   #4
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Do you have a solid floor or an air floor?
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Old 03 February 2008, 12:38   #5
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Hi. I have a solid floor. It's aluminum. I'll experiment with the throttle the next time I'm out. It's very dramatic though when it happens.
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Old 03 February 2008, 16:14   #6
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Spun prop hub?
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Old 04 February 2008, 01:50   #7
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You'd think a spun hub would show up trying to get out of the hole rather than just prior to (or just upon) planing. Though, I suppose, the engine power curve could play with that a bit as well.

jky
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Old 04 February 2008, 02:08   #8
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For many years we also had a Zodiac 4m SIB, one piece custom wooden floor, firstly with a 15hp Johnson then customised the transom to take a 35hp Johnson/Evinrude both tiller steer. For both motors we found the doel type fins simply fitted to the engine leg to work great in all aspects of the crafts use, especially getting out of the hole. At speed they also enhanced the stability of the SIB greatly.
I realise there's a school of thought on this forum that is strongly pro trim tabs and that's fine with me, but some of the opinions passed on the doel type fins cannot be supported by our 20+ years of use in all kinds of conditions, including striking submerged objects and high speed beachings. No broken legs or snapped metal etc.
I mention the above in detail because I am probably going to be the only one to heartily recommend you give them try. They fixed our 'holing out' with 10 mins work and no downside, the better fuel economy was a bonus too.
I am not against trim tabs, we've used them on the ski boats, but I am a long time satisfied user of the cheaper, simpler Doel fins on smaller SIBs and RIBs.
Cheers,
Paul
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Old 04 February 2008, 05:38   #9
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Quote:
Originally Posted by jyasaki View Post
You'd think a spun hub would show up trying to get out of the hole rather than just prior to (or just upon) planing. Though, I suppose, the engine power curve could play with that a bit as well.

jky
Possibly not. It never actually lets go properly in the beginning and will slip more as the load increases, so at the point the SIB is almost on the plane is likely going to be the highest drag (bow in the air & all that) Remember also that even a hard floor sib has a couple of bits of fabric acting as your planing surfaces, so the water will be turning them nicely concave at that point no matter how hard your keel is pumped...

Way to check - have a look & see if there's any rubber oozed out from the hub or prop body join with the rubber bit in the middle. Probably find the paint that it may have once had is missing too. I had this happen on an old 25. All that you were aware of was that it was a bit sluggish (felt more like a 15 than a 25 on hole shot) and didn't go so fast across the water although the engine was revving properly. Once a replacement prop was found it took off like the proverbial scalded cat.

If I can find the old prop I'll point the camera at it 'coz a pic has to be better than my dodgy description!
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Old 04 February 2008, 09:07   #10
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Originally Posted by 9D280 View Post
Possibly not. It never actually lets go properly in the beginning and will slip more as the load increases, so at the point the SIB is almost on the plane is likely going to be the highest drag (bow in the air & all that) Remember also that even a hard floor sib has a couple of bits of fabric acting as your planing surfaces, so the water will be turning them nicely concave at that point no matter how hard your keel is pumped...

Way to check - have a look & see if there's any rubber oozed out from the hub or prop body join with the rubber bit in the middle. Probably find the paint that it may have once had is missing too. I had this happen on an old 25. All that you were aware of was that it was a bit sluggish (felt more like a 15 than a 25 on hole shot) and didn't go so fast across the water although the engine was revving properly. Once a replacement prop was found it took off like the proverbial scalded cat.

If I can find the old prop I'll point the camera at it 'coz a pic has to be better than my dodgy description!
I've experienced this as well on a account of a spun prop hub. The other thing that I've experienced with my 3.4m zodiac which caused similar behaviour was an edge of fabric on the hull near the transom was lifting. This was a wide fabric patch that was added to reinforce the worn fabric under the transom where water had started seeping in because it was worn so thin. Anyhow, the water had pulled away a flap along the lead edge of this patch about 1/4-1/2 cm wide. You wouldn't notice it at a slow speed, but at planing speed you'd some sporadic cavitation. You might planing along OK, then come across a swirling current and the prop would lose grip on the water, the engine would rev similar to how a spun prop hub might behave. Double check the condition of he fabric along the bottom of the hull.
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