Rib Tender won’t plane two up

[girlofwight]

Good evening. Bit of background first, experienced boat owner - grew up on a boat yard 50 years ago, graduated from yard dinghy to a Mirror, then later in life my own larger vessels, first sail, latterly Hardy 42 now on 6th season and almost snag free.

However, always learning, so on my first rib... It’s a new 3.3m rib tender with a 20hp Suzuki on the transom. With just myself in it I easily got on to the plane, and bottled out at 25 mph (which is how the log is calibrated). It’s a bit of a play thing, intended to be quicker and easier to get afloat by myself for a pootle around.

With my partner on board yesterday I could not get over the hump and on to plane, alas.

I‘m keeping it afloat, so it was antifouled before handover. I collected it on lockdown day, so it spent 8 weeks on the driveway before being dunked, but I’m told the antifoul won‘t be the worse for that.

Three possibilities for the inability to plane:

1 - 20hp isn’t enough. I’m told that’s unlikely, and it should be fine.

2 - dirty bottom. See above re antifoul. The hull was AFd not the tubes, and the aft end of the tubes are in the water and very weedy. I did scrub them from afloat last weekend. I could lift her in davits and to a more through job, but as mother ship is berthed stern to its a faff.

3 - weight / trim. I did try altering trim on motor but no avail. We were both sitting on aft bench seat (the tender has a centre console). I could get my partner sitting forward alongside the console, but she is a bit nervous about ribs in general, and I don’t think the centre seat is as supportive / cosseting, less handholds no back rest.

The dealer is helpful, we are speaking on Tuesday, and suggestion is to fit a foil on the motor.

Before that call I’d like to know a bit more, so...

- anyone any experience of getting a small tender - 3m to 3.5m range - on the plane with 20hp two up? Any tips?

- thoughts on the foil?

- best way to clean tubes whilst afloat? Dealer says they would look awful antifouled. The brush I used before was quite soft, I’m going to track a stiffer one down. Do people find barnacles stick to hypalon in the same way as grp? Any other non AF solutions like a wax or grease?

As is often the way, there is a bit of emotional attachment here. After a difficult couple of years personally the rib was a present to myself, and although it’s small and basic compared to mother ship, I’m lavishing it with love as a little ship (ensign, lights, stopped short of go faster stripe) and I’m quite upset with this setback.

Any words of wisdom welcome.

 

[julians]

I'd have thought it should plane easily enough with 20 hp and 2 normal sized people, Assuming the engine is running correctly - which it should be if its brand new. we (as kids not adults) used to waterski with a 15 hp inflatable, 2 people in the boat and one in the water.

small ribs are very sensitive to weight distribution, too much weight at the rear will prevent planing (as will too much weight at the front - but this is much less common) so it may need one of you right up the front ,and I mean all the way up front, to get it planing - once its planing the person up front can probably move back a little. put the fuel tank up the front as well if you can - tie it using the painter line so it cant slide backwards. in my opinion weight distribution is most likely the cause of the inability to plane.

Another thing is to ensure the engine is at the correct height on the transom, I dont mean the trim angle I mean the height, too low and it will cause a lot of drag. you want it so that the cavitation plate is level with the bottom of the keel. to achieve this may require a block of wood on top of the transom for the outboard to rest on.

trim angle - trim the outboard all the way down/in and see if that helps.

Foils have always seemed to cause more problems than they solve in my experience - but you may find different

 

[BruceK]

Tender's are sensitive to weight at the back and I'd recommend you consider Nautilus smart tabs to help get you on the plane rather than foil. Foils can place a large amount of force on the motor bracket and transom where it hasn't been designed for such especially at speed when you dont need the lift anymore.

Smart Tabs SX - Nauticus Incorporated

Selector here : Smart Selector

verify from above link

Nauticus ST980-30 Smart Tab Trim Tabs: Amazon.co.uk: Electronics

or

Nauticus - Sx9510-30 Smart Tab Trim Tabs: Amazon.co.uk: Sports & Outdoors

 

[Dino]

I had a Caribe 12ft rib with a lovely 25hp Yamaha 2 stroke. There was a bench seat across the back with a steering console and if I was on my own in the rib I had to stand up and lean forward to get it on the plane. I fitted foils and it still didn't plane without getting weight forward. If someone sat on the bow seat it planed straight away. I think short ribs with decent sized/heavy engines struggle to push over the hump if all the weight is aft.

 

[BruceK]

I've had foil and smart tab. I found at high planing speed the tender with foil was susceptible to a sort of chine walk. Not enough to destabilize the boat but enough to make it unpleasant. That might have been peculiar to mine though and not necessarily across the board

 

[julians]

I've had foil and smart tab. I found at high planing speed the tender with foil was susceptible to a sort of chine walk. Not enough to destabilize the boat but enough to make it unpleasant. That might have been peculiar to mine though and not necessarily across the board

Ditto-foils cause all sorts of odd handling in my experience.

 

[girlofwight]

Thanks folks.

 

[Greg2]

We have an AB aluminium hulled 3.1m RIB with a two stroke Mercury 15hp (no centre console) and it planes with two aboard. As previously advised, we sometimes need to shift weight forward as she gets on the plane but once there we can resume normal positions.

 

[atol]

check the pitch on the outboard prop is correct for the engine HP,,if it is under pitched, getting up on the plane will be difficult even at full revs.
it may be that the prop has been replaced in the past with an incorrect pitch/too fine a pitch one for a smaller engine.
a more corse pitch prop gives more speed less HP, a finer pitch gives less speed more HP

 

[Rocksteadee]

check the pitch on the outboard prop is correct for the engine HP,,if it is under pitched, getting up on the plane will be difficult even at full revs.
it may be that the prop has been replaced in the past with an incorrect pitch/too fine a pitch one for a smaller engine.
a more corse pitch prop gives more speed less HP, a finer pitch gives less speed more HP

This is your most likely reason.
look at the prop. It will have a number. Eg 12x13. First number is the diameter, second is the pitch, this is the distance it will travel through the water in one revolution (ignoring slippage)
You would need to look for a smaller pitch which will have the effect like a lower gear. However this may cause over reving when only one person.
The only way to accurately gauge this is to use a tachometer on the engine whilst at normal loading to determine max revs at WOT.
As a rule of thumb each inch of pitch reduction increases max revs by 200

 

[Rocksteadee]

.

 

[atol]

This is your most likely reason.
look at the prop. It will have a number. Eg 12x13. First number is the diameter, second is the pitch, this is the distance it will travel through the water in one revolution (ignoring slippage)
You would need to look for a smaller pitch which will have the effect like a lower gear. However this may cause over reving when only one person.
The only way to accurately gauge this is to use a tachometer on the engine whilst at normal loading to determine max revs at WOT.
As a rule of thumb each inch of pitch reduction increases max revs by 200

with 20HP they have plenty of power, it sounds like they need a corser pitch prop to provide more acceleration not a finer pitch prop.
my 3.4m rib easily planes with 4 people in it and a 25 hp

another thing they might check is if there is water trapped in the hull,or a drain plug come lose,a waterlogged hull will definitly effect performance,and may not be noticeable without inspection out the water

 

[Rocksteadee]

with 20HP they have plenty of power, it sounds like they need a corser pitch prop to provide more acceleration not a finer pitch prop.
my 3.4m rib easily planes with 4 people in it and a 25 hp

another thing they might check is if there is water trapped in the hull,or a drain plug come lose,a waterlogged hull will definitly effect performance,and may not be noticeable without inspection out the water

A finer pitch prop, lower second number, will give more acceleration (required to get a boat planing) but it lowers max speed, think 1st gear on a car instead of second.
water logged hull can also have an extra loading effect

 

[Ferris]

20hp should be more than enough, my 3.1 Zodiac planes with two adults with a (heavy) 8hp Honda - although it needs weight forward to plane.

Just a thought - is there a cavity in the hull holding water? This would exacerbate the stern heavyness when going over the hump. I mention as this happened to me once. Also is there more weight you can move forward, like the fuel tank?

If you are going to play around with props you are going to need a tacho to read engine revs (if you dont have one already). You can get small cheap handheld ones

You can get clear, flexible antifouling for tubes, my recollection is that its not great but things may have moved on and it might suit you. Don't think its your main problem however.

 

[atol]

A finer pitch prop, lower second number, will give more acceleration (required to get a boat planing) but it lowers max speed, think 1st gear on a car instead of second.
water logged hull can also have an extra loading effect

a finer pitched prop will only help if the prop is over pitched to start with!

 

[julians]

jeeez - before you start messing with props and rev counters get the basics right, ensure the weight distribution is conducive to planing, and the outboard is mounted right.

It'll just be a simple case of weight in the wrong place in my opinion.

 

[atol]

jeeez - before you start messing with props and rev counters get the basics right, ensure the weight distribution is conducive to planing, and the outboard is mounted right.

It'll just be a simple case of weight in the wrong place in my opinion.

quite possible and good advice,but if that does not work see above posts

 

[BruceK]

The OP can get to 25knts solo. If the prop is too fine would not prevent him from getting on the plane 2-up at all. It would in fact aid him getting on the plane. Possibly if the prop was too coarse then he would struggle. But 25 knts at WoT is about the expected speed he'd achieve correctly propped so I very much doubt it has anything to do with prop.

 

[mainshiptom]

on my little rib 3.1 meter with 6hp i use an extension rode and move forward and it will go on the plane

 

[atol]

The OP can get to 25knts solo. If the prop is too fine would not prevent him from getting on the plane 2-up at all. It would in fact aid him getting on the plane. Possibly if the prop was too coarse then he would struggle. But 25 knts at WoT is about the expected speed he'd achieve correctly propped so I very much doubt it has anything to do with prop.

yes this would indicate not enough horsepower,and a very heavy rib which is possible if it has console seats and box in front with heavy laminate some of them are designed for a 40HP