Change prop or add hydrofoil

[dawktah]

Hello!

I have the following conditions on my RIB: Brig 450 HL, weight 264lbs empty, Evinrude 60Hp, 240 lbs, 13.25 x 17 prop. Rear bench seat. The boat performs as follows: Once placed in gear forward or reverse the boat moves forward pretty quickly for a boat without any added power. However, once I add power and if I add it quickly I get a lot of cavitation, and the bow goes up into the air and the boat is very slow to get on plane. Once it does the rpms shoot up to 3,800 and the boat goes extremely fast!!!, throttling back to 3200 the boat still carries a lot of speed. I added sandbags to the bow locker, about 50-80 lbs of sand and now the boat gets on plane quicker without the bow coming up. I still however get all the cavitation and the sluggishness with rpms in the 1,000 range. once on plane the rpms jump to 3,800 and I am flying!!! The speed for such a small boat at 3,800 is frightening, so I generally cruise at 3,000 to 3,500 tops when I need to get there and seas are relatively flat, with more power to spare! My dealer wants to install a hydrofoil to solve all my problems. I understand how it will improve the speed to get on plane and will remove my need to put sandbags in the bow. However, the period when I have all the cavitation and the engine is struggling, becomes very problematic when I am trying to go over a wave, when I am below planing speed.

All info says go to a 19 or 21 degree prop, since that will lower my rpm at WOT. Won't this make the boat go faster once placed into gear? Will this make the boat more responsive before getting on plane? Will hydrofoil achieve same outcome without changing prop? I'd rather avoid a prop change for "easy" when "easy" gets holes drilled in my cavitation plate. Also, if there is another prop change solution.

Thanks for any assistance anyone can provide.

--Chris

[Nos4r2]

What revs are you at at WOT?

You don't want the RPM at WOT to drop if it's within the manufacturer's recommended range.Your engine will be working too hard and you're right-at tickover you'll be moving faster.

Don't mess around with hydrofoils. They aren't the best solution. Try these instead:-

Smart Tabs by Nauticus

or you could always put a ballast tank in the bow.

[dawktah]

Originally Posted by Nos4r2

What revs are you at at WOT?

You don't want the RPM at WOT to drop if it's within the manufacturer's recommended range.Your engine will be working too hard and you're right-at tickover you'll be moving faster.

Don't mess around with hydrofoils. They aren't the best solution. Try these instead:-

Smart Tabs by Nauticus

or you could always put a ballast tank in the bow.

Never seen, WOT, not that brave!

[Nos4r2]

Originally Posted by dawktah

Never seen, WOT, not that brave!

Go and find some flat water and try it-it won't go past about 45 mph at a guess

[Biggles]

I am guessing but I think you need to use the boat a bit more. Get some experience with it. Be wary of yours and the boats present limitations but enjoy it.

If the engine is cavitating then its not set up right. But from what you say and looking at the boat I have a feeling your getting the correct performance out of it. Can the engine be lowered or trimmed in more ?

By the way, and not meaning to cast aspersions, but how much experience do you have with small high powered sea craft ?

[dctucker]

It seems to me I was on one of those last fall, but rigged with a 60 hp Merc 4 stroke. Surprisingly zippy little boat. I do recall there was a lot of bow rise with 2 of us aboard. I don't know that I ever got the boat to WOT, but it was a pretty bouncy day for a boat that size.

I think the fundamental problem is that the console is too far aft, so you need to have weight up forward (another person, or ballast as you've done). SmartTabs would be a huge help on this boat I think, far preferable to a hydrofoil, IMO. They would really help to eliminate the bow rise. I've got them on my RIB and think they're damned good. I read enough negative things about hydrofoils that I chose the SmartTabs easily.

As far as the propping goes, go back to the dealer and have them figure it out! From the look of the boat, you probably bought it brand new. If there's cavitation and other nonsense going on, then the engine setup is wrong and the dealer should fix it. Let them drive it at WOT and verify that the pitch of the prop is correct, and the engine height is correct. If it is actually cavitating when you open it up, the prop could be right for WOT, but the engine may be too high (?). Does it stay well connected in hard turns at any sort of speed?

That ETEC has a lot of low and mid range torque, so you may also just need to get some more experience with the boat. Again, use your dealer, have them go for a ride with you!

[dawktah]

Problem is the boat is in the Caribbean and I am in Chicago. Experience I have enough, cavitation I hope is the right word. Other small boats I have been able to give a little power and boat moves ahead steadily. If you give a lot of power and the boat responds with acceleration. This boat just generates a lot of water churning and no forward movement, cavitation??? Slippage??? Then as the boat gains speed and gets on plane everything feels fine, just very fast. Handles well, trim works well (with weight in bow) Boat dealer, stated boat was WAY TOO FAST! He was concerned about speed. Engine dealer solution...hydrofoil. Shaft length can be a problem now that it has been mentioned. Dealing with this problem is the goal of my next trip in a month.

edit: Thing is boat dealer and engine dealer aren't the same, in same building but that's it. They work together but aren't one in the same. All issues I am having now boat dealer is sending me to engine dealer. Everyone is nice and helpful, so far. The E-tec 40/50/60 is the same engine, only programmed differently, boat dealer said engine was too powerful...

[fjarvis]

Hi there,
I've got the BRIG F400S with an E-Tec 40 on it, however I have the jockey console. I've got the fuel tank under the seat and in the bow locker just my anchor, and a spare 12L fuel tank. She handles well and with the engine trimmed down there is just normal bow rise when applying power. The boat planes easily and even 4 up with camping gear, I can get her on the plane as long as i'm not trying to get on the plane going into the wind. WOT @ 5500rpm gives me around 36mph.

Hope this helps as it's a similarish setup!

F

[Hightower]

I would have a look at the engine height first and make sure it's not to high. Cavitation plate level with bottom of v to start with.

Once that's been established then I would check prop size as a too small a prop may well cavitate whilst accelerating fast (wheel spin for car lovers )

Once thee have been looked at, then the way the boat is balanced might well be a problem. Have you checked that there's not a couple of gallons of water in the bilge, it has been known you know!

After these three basic checks have been made, then look at other bolt on ways to make your set-up work (trim tabs etc..)

[ian parkes]

You have the same prop and similar boat power and size to me . Although from reading this it sounds like that boat is a faster hull as the best I have seen is 38mph.2 up.
I didn't have any problems axept the boat reached WOT and a little more too easily , and it was very lively and responsive to the throttle making it difficult for a beginner in any rougher water .
I fitted a 19" pitch prop and the boat is a lot easier to drive and cruises better etc . problem now is when loaded it won't reach max revs in anything but flat water , so am re pitching the original 17" prop to an 18" which is only costing about £34 .

[BBirnie]

As mentioned before, Smart tabs make a huge difference, easy to install and relatively cheap. They also allow you to plane at lower speeds.

[dawktah]

Smart Tabs look like a good choice , will look to make sure cavitation plate is at level (or below) of V of stern. How do the Smart Tabs effect towing? My next task is getting a clear understanding of whether I can safely tow the boat behind a 40' powerboat at 24 knots? I've been a sailor for over 15 years and power-boating is been only the last 5-6 years. I will see if I can get some ideas here before posting a new thread. My first step is figuring out how to reinforce the bow to withstand the forces of the towing.

My options are for local towing do nothing since I can get to all of the USVI and the BVI without that much boat speed. However, when I travel to the Leeward Islands I really need to cruise at +/- 24 knots to make the 85 nm passage in a reasonable amount of time. My wife and I aren't trying to buy an 8' RIB with a 9.9 - 15hp just for this, seems like a waste. Not to mention storing it when its not needed.

--Chris, no more vacations in the VI, moving there!

[reinier]

If the anti cavitation plate is too high, the boat will cavitate even more if you get on the plane. I think the problem is that all the weight is near de transom. If I were you I would take the boat out on the flats and play a little with it.

I had a 40hp Yamaha 2 stroke behind my Brig F360 (no console) which is only 175 lbs (80 kg) and with the right prop it hits 44 mph on the GPS, and the bow rises extremely high while getting on the plane. Took me a while before I could handle it, it's best to trim your motor in, to get the nose down while getting on the plane our cruising at high speeds. That's the way I learned to drive the boat. The padded v-bottem is a little tricky for beginners at high speeds!

You could also ask some people too cruise along, that will make your boat easier too handle. @3800rpm the boat won't go anywhere near speeds that will make a Brig F450 dangerous to drive!

[Chris]

Firstly it sounds like your engine is trimmed out if the boat if the bow is lifting up as you say so it is... make sure you are trimmed fully in when trying to get on the plane and that when trimed fully in the leg of the engine is parallel to the transom.

Secondly I am really confused that you are reaching "frightening" speeds at 3800 RPM as at that RPM I would be suprised if you are pulling more than 25knots. Again this makes me wonder if your leg is trimmed to far out and as a result the boat feels very loose on the water which would make the boat feel faster due to it being unstable.

Can you take a picture of the engine from the side when it is fully trimmed in and post it on here?

Also look at the height of the plate above the prop and compare it to the bottom of the transom it should be level with the bottom of the transom or maybe an inch or two above the bottom of the transom.

I would not go changing props yet and certainly dont bother fitting trim tabs until you sort out the rigging of the engine which is clearly not right.

[jwalker]

Originally Posted by dawktah

All info says go to a 19 or 21 degree prop, since that will lower my rpm at WOT.

Don't go fitting tabs or foils until you've got a reasonably running boat.

Do the checks on the engine trim as suggested. Be aware that most outboard motors have a pin through the lower part of the mounting bracket which limits the amount of 'in' trim which can be applied. If this pin is too far outward, even full 'in' trim can still be too far 'out'. I hope I've explained that ok. The pin can be moved to an alternative set of holes to make adjustment. The engine can satisfactorily be mounted with the plate below the bottom of the V if necessary, although folk tend to go for high positions generally, 25mm below the V is still a good position in order to bury the prop more.

However, if all engine rigging is fine and you are happy with your boat driving ability, the characteristics you are describing would lead me to reduce the propeller pitch. What you are experiencing sounds like the prop breaking away because it is too highly loaded and this causes ventilation (not cavitation). When/if the boat gets over the hump and planing, the load on the blades is lowered and it may regain its grip on the water. It may also account for you feeling the boat is very quick for 3800rpm. It could be you're closer to WOT than you realise.

The cavitation plate is properly known as an anti-ventilation plate. Cavitation is an entirely separate phenomenon.

To sum up, make sure the 'in' trim is correct, engine height is correct and then, if necessary, try a lower pitch prop.

[Nos4r2]

And nine months later, out popped another post

[Chris]

Doh! just realised.

[jwalker]

Me too...

That's what happens when folk don't have the courtesy to finish a post they've started.

[Pablo]

Doesn't matter, it was interesting reading for me. Thank you all.

[ian parkes]

so many post like this never have an outcome , everyone does thier best to help but we never know what the real problem was