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Old 25 July 2020, 12:59   #1
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Help needed re engine height and prop Ribcraft 5.3 /honda bf100

Repowered my 5.3m late last season and with a slow start this season ,i,m hoping for some helpwith this as my progress so far is pretty slow .,as above 2006 ish r/c 5.3 now with 2014 bf100 long shaft ,centre console boat 2 inline jockeys only planned for single use, first picture is as fitted by seamark nunn for me .
originally had 15 pitch honda 3 blade prop as engine was on heavy fishing boat
step 1 15 pitch was way too weak so opportunist buy honda 4 blade 19x12.5 fitted
fitted 4 blade and was great improvement 5700 wot and 37.3 knts into an outgoing river stour tide . thought i was all good !
went out to a very confused orwell /stour chop into a 16 mph southwesterly but felt performance of boat was not great ,seemed a bit bogged down at rear and seemed same coming back with a incoming tide (bear in mind single console is well forward so you seem to ride the rough fairly )
on return home had a look at engine position and instantly thought too low by 20mm (1st picture ) raised by one hole (athough i really fancied trying two ) 20mm and returned next day at similar state of tide to try same route .
max speed 34knts at 5500 wot but seemed a little more comfortable ,however there was some splashing left and right of transom ,hopefully can be seen on second photo
would seem to me that i havnt really got the results i expected from raising ,which has totally thrown me into the unknown ,any tips /knowledge greatly recieved ,nb yam 60 was 114kg and honda is 144kg i think
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Old 25 July 2020, 13:09   #2
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try to get wake up the right way
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Old 25 July 2020, 14:23   #3
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sorry trying too download pictures but failing
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Old 26 July 2020, 03:45   #4
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I'd say your initial thoughts on going up 2 holes were probably right. I know it seems odd you appeared to lose performance by going up the transom but that defies the physics, less drag = more speed. Cant explain the apparent loss in performance but you've nothing to loose in going up another hole.
Something says to me there was another issue at play on your second run with the engine higher.
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Old 26 July 2020, 05:23   #5
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Originally Posted by beamishken View Post
I'd say your initial thoughts on going up 2 holes were probably right. I know it seems odd you appeared to lose performance by going up the transom but that defies the physics, less drag = more speed. Cant explain the apparent loss in performance but you've nothing to loose in going up another hole.
Something says to me there was another issue at play on your second run with the engine higher.
Great thankyou ken ,what you say here is logic ,and i am sure your right about "another issue at play " ,so good to go for another hole upwards ,but still slightls unsure about fan shaped splashing either side of transom ,does that not signify prop too high ? ,i have read that four blade props create more lift and i did see evidence a litlle of the rear was less bogged down (maybe my imagination ) ,sort of makes me wonder if more lift on the rear into a big chop on a boat with the console well forward is reall a good idea ,
i also have a 17 pitch 3 blade stst prop somewhere from my highfieild with 90 honda ,so that should fit and worth a look
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Old 10 May 2021, 04:04   #6
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Hav,nt been out much with restrictions ,but see ing there seems to be several prop knowledgable people around at present may be some more input re this ,i have tried 21" prop now with discussion on recent post which i do not want to hijack
any comments especially re 3/ 4 blade advantages /disadvantages into rough seas would be especially helpful ,(also thanks BK your input was very helpful )
thanks
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Old 10 May 2021, 04:35   #7
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Bearing in mind that getting a prop “just so” is very much trial & error, especially when it comes to fine tuning those last few performance tweaks. But here are my thoughts.
The very first thing you need to do is establish a base line reference. Set the engine height so that the AV plate is level with, or slightly above the keel. This is with the engine trimmed so the plane of the AV plate is in-line with the keel.
Do some test runs in good, calm’ish conditions & check the WOT with your normal load. This should be within the ideal WOT range for your engine. Do a few runs with & against the tide, record your WOT/speed/trim.
If you have a selection of props, repeat the above with your different props.
Once you know where you are with your WOT, you can make educated guesses from there, until then it’s just guessing.
I’ve found that 4 blade props perform “better” on RIBs, this is subjective based on my preference for grip & handling rather than top speed. For me RIBs are the 4x4 of the sea, not the sports car. If all out speed is your thing, buy a speed boat (in my opinion)
Some broad generalisations about 4 blade props:-
They can run slightly higher than 3 blades without blowing out.
They use around 10% more fuel than an equivalent size 3blade.
They grip better in the rough & are less prone to blowing out.
You lose a couple of mph off the top end compared to an equivalent 3blade.

21” sounds a bit tall for your setup.
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Old 10 May 2021, 15:55   #8
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Originally Posted by Pikey Dave View Post
Bearing in mind that getting a prop “just so” is very much trial & error, especially when it comes to fine tuning those last few performance tweaks. But here are my thoughts.
The very first thing you need to do is establish a base line reference. Set the engine height so that the AV plate is level with, or slightly above the keel. This is with the engine trimmed so the plane of the AV plate is in-line with the keel.
Do some test runs in good, calm’ish conditions & check the WOT with your normal load. This should be within the ideal WOT range for your engine. Do a few runs with & against the tide, record your WOT/speed/trim.
If you have a selection of props, repeat the above with your different props.
Once you know where you are with your WOT, you can make educated guesses from there, until then it’s just guessing.
I’ve found that 4 blade props perform “better” on RIBs, this is subjective based on my preference for grip & handling rather than top speed. For me RIBs are the 4x4 of the sea, not the sports car. If all out speed is your thing, buy a speed boat (in my opinion)
Some broad generalisations about 4 blade props:-
They can run slightly higher than 3 blades without blowing out.
They use around 10% more fuel than an equivalent size 3blade.
They grip better in the rough & are less prone to blowing out.
You lose a couple of mph off the top end compared to an equivalent 3blade.

21” sounds a bit tall for your setup.
Thanks for your thoughts ,i think starting again is the best option ,i need to get my old clipboard out .i think i have genuine 17 pitch 3 blde ,19 inch honda 4 blade ,17 pitch mi wheel st st 3 blade and the mercury 21 3 blade
so i should be able to make some observations ,as you say its not about the speed mainly for me its about the see saw effect .
It sounds very simplified but i would like to somehow understand the props effects when hitting of a wave or chop ,and the bow go,s up and of course the stern goes down ,it has then been suggested that the rapid grip of the prop lifts the stern back up and this cycle continues to exagerate the see saw effect it was for this reason i had a go with the 21 prop as i thought it would not spin to much like a first gear ,and it did feel quite effective (or maybe i imagined this ) .
The honda also has a high torque B.L.A.S.T. low rpm feature which may be working against me in this
However if i am sounding rediculous re this ,please tell me as i am quite used to being told similar re the early onset d word
thanks
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Old 10 May 2021, 16:37   #9
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Thanks for your thoughts ,i think starting again is the best option ,i need to get my old clipboard out .i think i have genuine 17 pitch 3 blde ,19 inch honda 4 blade ,17 pitch mi wheel st st 3 blade and the mercury 21 3 blade

so i should be able to make some observations ,as you say its not about the speed mainly for me its about the see saw effect .

It sounds very simplified but i would like to somehow understand the props effects when hitting of a wave or chop ,and the bow go,s up and of course the stern goes down ,it has then been suggested that the rapid grip of the prop lifts the stern back up and this cycle continues to exagerate the see saw effect ....


Ahh! Sounds like you’re porpoising, I wasn’t aware that you had this problem. Usually happens on stern heavy boats trimmed up too high. Where is your bottom stop bolt on the engine? Is it in the hole nearest the transom? Have you got any aside on photos of the boat on the plane?
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Old 10 May 2021, 16:40   #10
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Have a read of this:
https://continuouswave.com/whaler/reference/prop3.html

The article explains how stern lifting props work and is very interesting. Getting the right design prop (and thrust shape) seems to be what you are after for a smooth ride and it will probably be a 19" pitch.
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Old 10 May 2021, 17:11   #11
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Ahh! Sounds like you’re porpoising, I wasn’t aware that you had this problem. Usually happens on stern heavy boats trimmed up too high. Where is your bottom stop bolt on the engine? Is it in the hole nearest the transom? Have you got any aside on photos of the boat on the plane?
Thanks ,not sure which bolt you refer too ,does picture at start of this thread give the answer ? thanks
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Old 10 May 2021, 17:21   #12
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Have a read of this:
https://continuouswave.com/whaler/reference/prop3.html

The article explains how stern lifting props work and is very interesting. Getting the right design prop (and thrust shape) seems to be what you are after for a smooth ride and it will probably be a 19" pitch.
Thanks for the bed time reading ,thats pretty heavy for me athough it does help my confidence re the d word ,its good to be taken serious ,infact it does remind me why i put the photo,s of the wake on the original post ,i am sure there are wise people out there who could look at that wake and tell me whats going on . help is appreciated so far thanks
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Old 10 May 2021, 22:06   #13
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Always a bit of work getting the best from any setup, most manufacturers fit engines to low as they give good general handling. Most boats I have had have benefited from 4 blade props (including a couple of Hondas). Hondas aren't overly gutsy engines in my opinion to start off with. A 4 blade will give you better stern lift, hole shot and general midrange performance but will often drop a little top end speed.

A jacking plate is an option if you have the spare cash, as these let you adjust for different applications.

Check the mounting positions from this mercury screen shot, you will see theres really no right or wrong, its more to do with what you want to achieve.
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Old 11 May 2021, 03:19   #14
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Thanks ,not sure which bolt you refer too ,does picture at start of this thread give the answer ? thanks


Yup that’s the one, looks ok.
Re engine weight, your Yam 60 was around 120kg, the BF100 is around 165kg so you’ve added around 45kg on the transom. This has altered the balance of the boat & is probably a major contributor to the porpoising. Try putting a 25kg bag of sand in the anchor locker.
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Old 11 May 2021, 03:35   #15
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This has altered the balance of the boat & is probably a major contributor to the porpoising. Try putting a 25kg bag of sand in the anchor locker.
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.
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Old 11 May 2021, 04:09   #16
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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.


I’m not a fan of Doel Fins, they seem to have a place on some smaller engines that lack adjustment, but generally on larger outfits, the problems can be dialled out.
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Old 11 May 2021, 04:20   #17
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I’m not a fan of Doel Fins, they seem to have a place on some smaller engines that lack adjustment, but generally on larger outfits, the problems can be dialled out.
I've only been out on the boat less than a handful of times. I've given it a fair crack and gave it a chance, now it's time for it to go. My new Solas four blade gives stern lift. Very pleased with it.
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Old 11 May 2021, 09:20   #18
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Yup that’s the one, looks ok.
Re engine weight, your Yam 60 was around 120kg, the BF100 is around 165kg so you’ve added around 45kg on the transom. This has altered the balance of the boat & is probably a major contributor to the porpoising. Try putting a 25kg bag of sand in the anchor locker.
Yes i think theres alot of mileage in the weight change ,i will try the sand in the anchor locker ,i would also say that the see saw balance point is on the second seat when two on board ,where somebody of 100kg can sit and ride as smooth as it could poss get , it seems slightly strange that an increase of 45kg would appear to have a bigger effect than a second person or indeed the fact i go at 110kg may all be relevant ,i have even wondered if when these boats are powered at the top end of their quota they may do better with further rearward double console type layout ,still despite buying a dual console i must remember i bought this as a single user boat as i couldnt find a 4.8 and the console is same as a 4.8 and is very forward
I suppose you could suggest that i bought the boat as a single person wave jumping boat so i should be happy ,i suspect truthfully i just fancied selling one boat so wanted one boat to do all and seeing i spent alot on the engine and the top brand ribcraft this should be the choice ,however real expierience is telling me otherwise
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Old 11 May 2021, 10:02   #19
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When I re-powered our RIB (40hp to 50hp) I was able to move the 25l fuel tank forward to compensate but if it had been any heavier I would have been moving the entire jockey seat and console forward.
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Old 11 May 2021, 17:25   #20
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As mentioned, I have 4 blade props on most of the boats I use (my own and the commercial ones I skipper). On my boat it gave extra stern lift and stopped porpoising. I also raises the engine two holes. Raising an outboard normally reduces porpoising, I find it hard to believe the extra weight is the cause of the effect as I will often have two large people and even a 60lt live bait tank at the back of mine, ribs are load carriers after all. Tweak the height and experiment with props.
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