Return of the Doel?

[Limecc]

Originally Posted by Limecc

I currently have porpoising on a flat river with a Doel fin fitted, I'd raised the engine one hole, there was no ventilation. I wasn't at the helm though and don't know what the trim was. Must get it removed and retry, it was on the engine when I got it.

I like that it seems to reduce displacement wake but what good is that if it causes porpoising? Before I raised the engine it was impossible to trim the prop to the point of ventilation when on the plane because of the flow of water over it. It acted like a trim tab in reverse.

So I tried the boat today without the fin and it was almost a disaster. Here's how it went. Fyi it's 4.6m long with a 50hp Suzuki and 13p stainless four blade Solas. Also a 18kg aux and 25l fuel tank and spare anchor right over the stern and there's another 25l fuel tank up front.

Loaded with four persons (300kg?), two just in front of the helm and one behind the helm on the stern seat, it just pointed skywards. Couldn't even get up on the plane and past 8 knots. Trimmed fully down. Really disappointing. Embarrassed!

Some readjustment of passenger weight enabled it to reluctantly plane. Would easily drop off again. Back to port where the passengers disembarked.

Went out on my own and it was much improved. It was more reluctant than I would have liked getting on the plane, but when up it was way under-propped quickly pulling max 6500 rpm at 27.5kn trimmed out. I calculated 11% slip where I would have liked to have seen 6%. Throttle backed off to 6000rpm, the speed was 25kn, 10% slip. Sea was not completely flat, was wave jumping. I think with two of us it would be propped about right.

I'm thinking of re-fitting the dreaded fin. Your thoughts and what to try next please? I think I know the answer, it's 2' too short and stern heavy. Need the extra lift the Doel provides to compensate. Maybe I should go up to the last engine hole, but would risk ventilation. AV plate currently 1" above keel.

[paintman]

You need to do all your tests with the 'normal load' you would have on the boat & make any alterations to achieve optimum performance in that state.

Increasing or reducing the load will affect the performance & there isn't a lot you can do about it - other than repeat the above paragraph.
If they came with a multi speed gearbox then you could select the appropriate gear(s) as you do when driving a car.

If a fin/plate of some sort provides the solution for your boat/engine combo then fit one.

[beamishken]

Simple answer is if the fin works for your application then use it. Forget what the naysayers say about them they help in some circumstances but not all.

[jonp]

Personally had very bad experiences with two boats I owned that I bought with outboard fins. On both boats in adverse conditions side on to large seas the outboard fins exaggerated the side leaning effect to the point of almost capsizing. Im not a novice to boating and drive commercial fishing vessel most days as well as a marine rescue volunteer. A better bet for stern lift are trim tabs, which don't have to be overly expensive, some like the Bennette slt tabs are cheap and effective and can also be setup to compensate for listing boats from prop torque or uneven loaded boats.

Away from that though, you don't mention the mounting height of your engine as this can have a big influence on porpoising. Often you will find the higher you move your engine the more it will reduce the porpoising.

The prop you are using might actually be working to well when trying to get onto the plane. If it is gripping to well and the engine to small for the load you might find a three blade more effective to get you out of the hole. In comparison I had a Yamaha f70 with a solas 13x 13 three blade on a similar sized rib to yours but when I tried a 4 blade had the same issue when the boat was loaded heavy.

The Suzuki 4 strokes are a bit sluggish in my opinion, having owned two 90 4 strokes and now a 140. The fact the rev limiter should of kicked in before you hit 6500 seems odd but Im not every familiar with that engine.

Take a look at the mounting height positions from this image.

[Matt]

Those slip numbers don't add up. Achieving 6% on a conventional monohull, ali prop and those speeds is virtually impossible. Could be your rev counter is a fair bit off, or prop pitch is an inch or 2 off what's advertised. Makes no odds for comparitive purposes on an otherwise like for like basis, but 6% is ridiculously low. I'd expect a 100mph stepped hull skater catamaran to be in the 6% range....

Either way, something doesn't seem right does it. a 5m? hull with a 50 should get on the plane easily enough 4-up and fully trimmed in it should be able to give you enough stern lift to be usable.

[Limecc]

Thanks for all responses so far.

@Jonp, you missed it but the AV plate is currently 25mm above keel with one possible hole left to go. Also the rev limiter is set at 6500rpm for this engine. I'm not a fan of trim tabs which won't help me getting out of the hole (at displacement speed) so not looking at those for now and we never go out in difficult seas, however your Doel experience is noted. Thank you.

@matt, I was hoping for 6% after seeing that's what Beerbelly and Sark are getting in this thread: https://www.rib.net/forum/f8/115hp-e...ots-86067.html

It may have been 10-11% because of the sea conditions jumping waves, I'll check it again on calm water. I'd expect the slip to be lower than the OEM prop, that is, 11.1/2x13 four blade stainless vs 11.3/8x14 three blade alloy. The difference between 6% and 10% is just over one knot.

@Beamishken and Paintman, I refitted the Doel as you said and intend to re-evaluate it. Based on my experience so far, except the porpoising (and I wasn't at the helm to experiment with trim), it's only been a positive thing so far.

[Orwell boy]

A friend of mine had one of these Ranieri boats ,but have a hunch it may have been 17 foot ,he had yamaha 70 on it but uprated to yamaha 100hp ,but sold it soon after ,not sure why .
i will speak to him and see if he has any help to offer ,he is a very sports boat aware person so might be useful

[Limecc]

Thanks OrwellBoy will be very helpful.
I saw this 17' Ranieri for sale in Portsmouth and notice it has both trim tabs AND Doel fin. Strangely looks like the fin is fitted backwards compared to mine.

[jonp]

I wouldn't agree that a 5m boat should get on the plane easy enough with 4 people onboard with just a 50hp as well as an aux on the stern. Thats a very small engine in my opinion and would need a pretty small pitch prop. With my small light rib and a 70hp I needed two different props depending on what loads I carried. I had a quick removable prop lock that allowed me to swap props depending on how the boat was being used on any given day.

That boat with both a fin and trim tabs was obviously not designed to have that much weight on the stern if it needs those added on. The Suzuki 70 is a heavy lump its self, let alone the aux and possibly dual batteries and fuel at the bum end of it. Id have a bit more faith in the main engine and do away with that aux for a start. Ive owned boats for 43 years and never had a breakdown or problem where Ive needed assistance to get home, I also work commercialy on outboard powered boats daily.