Props, Trimtabs and Foils

[Traveller]

Hi all,

Looking for advice for the new boat/setup, as far as props pitch/size/blade and if I should consider putting on either trim tabs or foils.

Stessl Pro Tournament 475
Honda 75Hp 4 Stroke

Currently thinking about Solas SS props, but can't make up my mind on which ones are more appropriate. Will be talking to Solas more next week but I thought I see if anyone here willing to share their wealth of expertise and knowledge, and provide some sagely advice.....

Thanks....

OK lets break it down into manageable chunks...

Props

Stainless props on river boats isn't such a good idea - one good thump on a rock or log and it is possible you will destroy your gearbox - whereas with a alloy prop - more likely you will just bend a few blades.

That said I have a SS prop on my river boat and like it a lot.

Better bite - better top end, better fuel efficiency, less drag etc..

Do I pucker the sphincter when in dirty water in a river and theres a chance of hitting a log? You bet!

Everythings a trade off in life, ya just learn to live with the risk (and be a lot more carefull than otherwise)

Trim Tabs

Does your boat lean to one side or the other on plane? (chine walking?)

Then trim tabs willl help keep her level side to side against say a wind - this can be an issue with boats that have say a bimini or 't' top where the wind can heel a boat over quite a bit due to the sail cloth area of the top acting like a parachute.

Usually it's not such an issue in small open boats, often just redistributuion of onboard weight will achieve the same effect.

Trim tabs CAN be used to push the bow down into chop - forcing the bow to cut the waves rather than bump un n down over them.

That said a pair of transom wedges can also achieve this effect, as can a lot of "cup" in a prop blade!

Foils

There are different types of foil - like the hysucat foil (a boomerang shaped aerofoil wing fitted between two sponsons on a cat hull) or the outboard 'cav plate' mounted 'whale tail' type.

Usually applied to the vent plate (often erroneously referred to as the cavitation plate) to correct a "problem". Also known as whale tales, Dole Fins and other descriptions.

Often underpowered boats will benefit by increasing planing surface - getting a slug of a boat out of the hole & over it's own planing edge easier!.

Also - porpoising can sometimes be benefited with fitting a Foil..


In order to do anything to your boat in as far as "improving it"...goes - specially with regard to props - you do need to supply some hard data to start with based on what prop you already have.

Basically we need to know...

what boat you have
what it weighs
how long it is
what degree V is has
What OB you have
The OB gear box reduction ratio
What prop you have (Brand, material, pitch & Diameter)
The OB's Manufacturer Specified WOT (wide open throttle)RPM's range
What speed your boat does by GPS at WOT now, and what RPM's the OB turns NOW woth the prop you already have!.
What prop slip factor you have now.
What Gallons per hour your burning at idle cruise and WOT rpms (for which you'd need a fuel flow meter!)


There is a computer "prop" simulator program I could send you by email to 'play with' if you like, but unless you know the answers to most of the above questions, you won't know what figures to put into the simulator in order to be able to get any meaningfull data out of it again.

Basically - if you have the above data - and you plug it into the program - you can 'fudge' the "prop slip factor" a few % points either way until the simulator is duplicating your real world performance with existing known prop - once you have that - then you can TELL from the program what would happen if you increased or decreased pitch of prop, or altered diameter, or numberof blades and so on and so forth...

Solas from emeory have a similar "program" on a set of xl tables printed out for their props - with "average sized/weight recreational boats as a 'rough guide' to prop selection from memory. The pogram I have is a 'merc' one thats a little more flexible IMHO.

Unless you have a basic understanding of props and their selection performance and how to calculate things like % slip factors - then really - your going to have a tough time selecting the "right one".

It ain't exactly rocket science or alchemy but sometimes can seem a little like it.

Cheers - hope this helps.

[Traveller]

trouty,

I haven't got the boat yet. I'm just trying to make sure I get the setup right from the word go rather than playing around afterward and spending more money...

I've been given a choice of Solas SS props.. I just don't know which one is going to be the right one for the boat... as the dealer involved is not used to setting type this type of boat I'm concern that they might pick the wrong type that won't give it its best performance.....

With the trimtabs... a few friends in the US kept telling me to add gas filled auto trim tabs like Nauticus or Bennets, and jacking plate to the boat... I haven't seen bream boat in Aust with this stuff on it... so I thought I ask the question....

If the Honda's anything like my Merc - you should be able to buy a workshop manual for it, (not the owners manual that comes with it).

Usually inside that is a table of the props that will suit the OB and which one suits what length & weight boats!

This is how the dealer selects a prop to get you started.

As for Jackplates - if you run in very shallow water, then yes a jackplates handy - but they are a couple grand by the time you get one new landed here (Solas agents in WA also have one brand of jackplate (panther?) in stock from memory).

Also - they can cook a motor in 2 seconds if you lift it so high that it's water inlet loses it's prime... Normally the jackplates are fitted to flats boats for shallow draft and they also fit at the same time a low water pickup kit to the front of the gearcase so that you are less likely to overheat the engine by raising the water inlet above waterline. As well they normally fit a trim indicator gauge so you can tell how far you've lifted the jackplate withoiut having to look over the transom while on plane, and water temp and water pressure gauges so you DON'T lift it too high and cook the engine for want of cooling water to the water pump impeller. All up likely a decent hydraulic jackplate with all the other fruit would cost around the 4 or 5 grand!

Then also, they don' seem to manufacture low water pickup kits for outboards much below 150hp in size so sticking a jackplate on a 75 Honda if you can't get it converted with a low water pickup kit means you can' use the full potential of a jackplate...

When they Are set up right - you can run as shallow as wet grass!
(Kidding - about 4 - 6 inches draft on plane).

As for the self levelling trim tabs - well - really we are talking about toys for much bigger boats than a little alloy bream boat to be truthfull...

How much room have you got on that transom - if you ended up with a dole fin on the outboard cav plate, plus a couple transducers and a pair of 12 inch bennet tabs on the transom - heck it would be like the aelerons on a jumbo jet wing back there!

Just lots more "sharp edges" for bream to cut your leader on when they run you round the back of the boat!

If you want Bennet tabs, then maybe I can get you a discount, but I really think they would be overkill on a small bream boat...sure they are a nice toy to have - but will you ever really use them (considering the cost - at least another 3 or 4 grand!).

I mean - with all a these "add ons" - your talking perhaps another 10 grand on top of the boat cost!

Probably best to get the boat - see how it performs and then go from there if you have any identifiable problems IMHO!

With smaller boats usually you can move a passenger or gear around to level out the ride - without needing trim tabs etc. Once you get into a 25 fter weighing 4 tonnes - shifting a person around doesn't afect the trim all that much, hence people revert to trim tabs to do the trick.

You'll find that, your power tilt / trim - will be able to raise or lower the bow a bit (if you have a prop with good grip and enough cup) so trim tabs are probably a little bit superflous for that use - and like I already said - moving yours or your fishing partners backside a foot or so to port or starboard will likely 'level out' any side to side lean easey enough.

I find in my own boat at 4.85 meters that if it's not trimmed right (level side to side) - I get a sore back. Now trim tabs only level the boat while your moving (not while drifting or at anchor or on the electric troller for example), so they are only of any use while moving - not once you shut the engine off whereas if you shift gear or assengers around to get the boat level to start with - then you don't really need the auto leveling trim tabs...while on the plane or at rest coz the boats already level!

Where they can be of use, is in travelling long distance in a short chop - where you can put both tabs down to lower the bow of the boat and make it cut the waves with it's bow rather than slap up n down over the chop.

In this instance they can indeed be a help.....but your not likely to be doing any 50 mile offshore runs in a 4.75 stessle bass boat are you?

If you wanted to be able to pull the bow down without trim tabs - you COULD install transom wedges to increase the fully trimmed down angle of the prop...pushing the bow down into the waves in the process....

This is dangerous on some hulls - as it leads to "Bow steering" where the bow will follow a boat wake or wave trough in a following sea and THROW, the vessle hard to port or starboard with very little warning...so the use of bow down (by transom wedges to increase OB down trim angle & application of full engine down trim, ) needs to be used with REAL caution.

Very first time I mounted my new (to me) 50 merc 4 stroke tiller steer to my 4.85 alloy hull - I did what most guys do and gave her a squirt at WOT to see what she would pull on the GPS. Not knowing any better I applied full down trim on the OB, and when I hit a boat wake, it veered so hard to port, that it almost thru me out of the boat - only the tiller handle saved me - and even then, it made the situation worse because it made me pull the tiller handle to starboard which turned the boat even FURTHER/HARDER to port. I was bloody lucky not to be thrown out of the boat...as it was my azz slid across the thwart seat about 2 feet and the boat chine very nearly dug in and flipped - it was bloody close - close enough to leave skid marks in the undies and to get me to slow down a little until I figured out what I was doing wrong!.

So - if you chose to use transom wedges - for even MORE bow down attitude, (instead of trim tabs) - you would need to make sure you applied plenty more upward trim angle to the OB while running downsea or across boat wakes etc (and wear ya deadmans lanyard - which I wasn't!)

Basically I reckon If I were you - I'd get the boat delivered - get some hours on the engine - so it's run in and thru it's first 20 hours service etc, THEN I'd start to look at what prop might be the best one - using the figures you'll then have available to you...and if you want we can run them thru the prop calculator..

My best guess is that you'll want to start out with a 14 3/4 inch dia prop with about 15, 17 or 19 inches of pitch - but that can depend on the weight of your hull etc

Anywhere in that ball park should get you started with the rig you have...

Alloy props are only about $100 odd bucks - usually you can beg borrow or steal a few from the dealer (or solas) to test out to get optimum propping.

You should remember - that over time as you add weight to the boat with all the essentials you find you "need / can't do without", that the prop pitch MAY need to be dropped two inches or so, thus getting a boat propped 100% correct on the day delivered is seldom the right pitch 2 months down the track when you then have a spare tool kit, spare fuel, a beach umbrella, 100 litres of water in the livewell and two extra passengers on board...

Sometimes it's "best" to get ya boats weight issues settled with it's "normal" running load, before you get too technical on the props pitch issues...

It seems theres always things you "add" to a boat as time progresses, specially during the first 12 months, so I wouldn't get too worried about getting the prop dialled in on the first day.

Most owners manuals tell you not to run WOT during break in or the first 20 hours or so - and you need to run WOT quite a bit to get a prop dialled in so this can't practically be done untill you've had it a couple / three weeks or so...

To give you an idea.

An extra 2 inches of pitch in a prop - will normally lose you about 150 - 200 rpms in WOT rpms & gain you about 3 or 4 knots...

Is it going to matter to you - in the first 3 weekls while breaking in the OB whether you can do 35 knots or 37 knots, if you aren't sposed to be running @ WOT anyway?

I reckon whatever prop they supply will likely do you OK for a few weeks at least unil you need to start messing with optimum prop pitches / blades etc

Thats how I see it anyway - just holler if you want the prop calc program (send me your email addy by PM). Its a zipped file so I can send it along by email no worries.

Cheers!

[Traveller]

Hi Trouty,

Thanks for the additional info. I think you're right about the transom wedge and trim tabs...

With the props I'll talk to solas about loaning me a props to test out first, they seem to be ok with this.....

Thank you for your insight.. really appreciated... I'll send u an email with more questions once I got the boat...

But at least I'm happy to learn that I don't have to get a trim tabs and don't have to get jacking plate as well.....

Now just have to get that props selection right...

One quick question, does foil lower your top speed or give any performance degradation?

Hi Traveller.

A foil will typically cost you a couple or 3 knots of top end due simply to increased drag.

If for example - you had an azz heavey boat with a big bloke like me down the back on the tiller, and no one else on the boat to add weight up front - to keep the bow down while trying to get on plane, - the foil might help hole shot a lot by providing more lift - and planing surface (length) further back behind the transom.

They can have advantages - in those sorts of situations - but if you have a well balanced boat driven by say a wheel on a centre console - and you have enough HP (which you do appear to have) you shouldn't need a foil - unless you find that you do have a problem that needs correction.

Same thing with the jackplate and trim tabs - if I had the extra cash laying about, I'd have both on my boat tomorrow with no hesitation - but, if spare cash was tight - then I wouldn't consider a jackplate unless I fished the flats a lot and needed the shallow draft...

I guess it comes back to where you fish - and what the waters like - if it's extensive flats and thats the sort of fishing you love doing - and the extra cash isn't a problem - then by all means go for jackplate!. They are a reallly great fun item to have - they can add a few knots extra top end to your WOT speed (due to reduced gear case drag)...

Theres a heck of a lot to be said for "he who dies with the most fishing toys wins" appraoch to boating if thats what floats your boat so don't let me talk you out of these fun toys if your the sort who can afford it and loves to have all the bells n whistles - I can tell you for 100% positive that if cash wasn't tight right now - I'd have the lot in the blink of an eye!

The difference I guess is this...

You probably won't NEED any of those things on your boat - but she'd be a hell of a rocketsled if you DID have em!

If this boat is your fun / stress release etc - then hey, blow the budget and get the lot (then swing by n pick me up so I can have a play with em too!)

Good luck with it - by all means email me - or ask questions here so everyone can learn - and just holler if we can run some prop numbers thru the prop calculator program for fun after the first 10 - 20 hours.

Cheers!