BF30 on a 13' Boston Whaler - slow with new prop

[ztrain727]

Hi Folks -

As the title states I have a 2001 BF30 on a 1963 Boston Whaler 13'. Boat weighs around 600lbs with outboard and gas. It came with a 9.9x9 pitch prop and tops out at 23-24mph, 6200 rpm.

I bought a 9.9x13 prop which is only 1" up from the stock 9.9x12 and I can only reach 4700rpm, 22mph and that's with only me in the boat plus some gear.

Is it possible they manufactured a lower gear ratio version of this motor (it came from a big river raft)? Or is there something else that could be wrong with the set up?

AV plate is just out of the water at WOT. Carbs were cleaned recently and looked good.

 

[SkiDad]

I would go 11 pitch if there is one

 

[steelespike]

Is your speed by gps? The calculator says only 5% slip at 24 mph.About 12% is more typical.
Calculate for rpm at 12% is 6600. I do suspect that there is a problem with the motor If 6200 is accurate and it needs a 9 to reach it.
Charts say the BF30 are 2.08 ratio.wot rpm is 5700-6200.
You can manually check your ratio pull the plugs and turn the flywheel by hand just over 2.00 for one of the prop.
I'm wondering if your rpm is higher than 6200.It would seem that the 9 would yield more than 6200.
Your 4" increase in pitch would knock your rpm down. You should probably verify the tach is accurate. be sure the motor is in excellent tune.
I think it is more significant that a 13" is only 1" down from the highest pitch than only 1" higher than the "stock" 12.
The 9" is close to what would be used for a really heavy package.

 

[Sea Rider]

Gee, another prime example going + 500 revs with one more pitch size while maintaining same prop diam size. If 6200 is the max wot rpm for that OB, you're right there while lightly loaded and solo boating. Yo have an engine/transom height isssue, the idiotic rule of thumb "AV plate even with hul " is killing the top performance you can get out of that 30. There's a miss match between lower leg WD plate and water flow passing at speed,

While AV is being hightly exposed prop is loosing best prop grip & thrust and if badly trimmed much worst. Will assure that at tight close turns while on plane that prop will aerate badly, right now only good for straight water courses on flat water. Personally would stay with stock prop along a engine height optimization, going one less pitch will shoot rpm by another 500 rpm, only good if boating medium to heavily loaded to compensate rpm loss with stock prop.

Post a side pic of your engine showing lower leg's both plates to have a look.

Happy Boating

 

[steelespike]

Antivent plate "about"even with the bottom is called a "rule of thumb"because it is a starting point. Thus the term " about".
Depending on the boat design typical load ,prop design, conditions and the motor design The motor may end up higher or lower than the rule of thumb.
The main reason for reference to anti vent plate is to see that the motor is at reasonable height.
I usually suggest raising the motor to a point just before ventilation becoms a problem.
Many boaters will tolerate a little venting if it gives them a little edge on the speed.

 

[ondarvr]

Antivent plate "about"even with the bottom is called a "rule of thumb"because it is a starting point. Thus the term " about".
Depending on the boat design typical load ,prop design, conditions and the motor design The motor may end up higher or lower than the rule of thumb.
The main reason for reference to anti vent plate is to see that the motor is at reasonable height.
I usually suggest raising the motor to a point just before ventilation becoms a problem.
Many boaters will tolerate a little venting if it gives them a little edge on the speed.

Very good answer.

 

[ondarvr]

You said the boat weighs 600 lbs, did you weigh it, or is that the listed weight? A BW can take on a great deal of weight as the foam absorbs water, so being as old as it is it could weigh a great deal more.

Also, what comes stock on the motor as a prop means nothing when finding the right one for your application, they have no clue as to how the motor will be used or what it will be mounted on.

 

[ztrain727]

Thanks for the thoughtful responses! I do think the motor is maybe a bit high which does effect my trim angle. Still, before I built a jack plate I had a long shaft on a 15" transom and was only going 1-2mph slower so I'm not convinced honing in the perfect prop depth is going to get me up to the 30mph range typical of this setup. Furthermore, I would think ventilation would cause RPMs to spike not drop. That said I will definitely pull the OB off and drill some holes a few inches lower and give it a try. Will also manually verify the gear ratio - thanks for that tip! As for the boat weight, I weighed it on its trailer at 840lbs so 600 was an estimate but not totally blind. I still plan to go weigh the trailer alone. Me and my girlfriend can lift/drag the boat with the motor off so it couldn't be too waterlogged. It has without a doubt taken on water at some point in its life - we just sanded down and reglassed some old hull damage and the drain plugs. Is it possible that the motor has simply lost enough compression to be equivalent to a smaller hp OB? It's definitely been heavily used, though it was rebuilt not long ago. I cannot add a photo right now because I only have phone access to the internet but I will next chance I get.

 

[Sea Rider]

I'm laughing at this point, IROT even whith hull is not a starting point, guess what, is not stated as so on any engine brand owner's manual that have the trouble to fully read regarding each own brand installation, all states 2 to 4 inches under keel/hull, why keep posting as so ?

That won't say a thing either as all engines although being mostly S-L varies its lower leg lenghts from brand to brand and all transom although are usually S-L doesn't have exact lenghts as you can have 37 to 40 cm for S and 50 to 55 cm for L lenghts. Besides once combo raises out of the hole engine/transom/hull parameters will vary. You can oly dial sweet engine/transom height by water experimentation, not while a boat sits out of water with engine hungs on transon, no matter which AV plate/hull bottom height dials.

Will second " they have no clue as to how the motor will be used or what it will be mounted on" will disagree on " what comes stock on the motor as a prop means nothing when finding the right one for your application" happens that "stock props delivered with OB's" which are "medium" pitched does very well their homework on light to medium light applications provided that engines are at least 70% HP rated than its max, from stock prop can go to a immediate prop maximization to better suit your needs, only if needed.

IROT = Idiotic Rule of Thumb

Happy Boating

 

[ztrain727]

Right, I think I need to play with the motor height. But regardless it seems I have a problem with my OB if it can only push a 13' hull to 4700rpm/22mph. I can't imagine I'd gain 1000 rpm with heigh adjustment, and even then I would be at the OBs bare minimum.

The tach seems accurate - trolling at idle I'm getting about 900, full about 6200. But even if those numbers were way off, it doesn't explain why a 13 pitch prop is going slower than a 9 pitch for a hull this small and light.

I definitely can try an 11 pitch prop but I'd be running a lot less pitch than most folks with the same hull and still less than the stock prop.

 

[ondarvr]

Right, I think I need to play with the motor height. But regardless it seems I have a problem with my OB if it can only push a 13' hull to 4700rpm/22mph. I can't imagine I'd gain 1000 rpm with heigh adjustment, and even then I would be at the OBs bare minimum.

The tach seems accurate - trolling at idle I'm getting about 900, full about 6200. But even if those numbers were way off, it doesn't explain why a 13 pitch prop is going slower than a 9 pitch for a hull this small and light.

I definitely can try an 11 pitch prop but I'd be running a lot less pitch than most folks with the same hull and still less than the stock prop.

Again, no matter what some people think or say, the "stock" prop pitch means nothing, experimenting and adjusting is the only way to determine what works best for how you want to use the boat.

If you have other BW owners with the exact same boat/motor combo (not just similar) that have shared their results with you, then that's good info and at least a place to start. Without that input there is no way to say what is right, only testing will show what's needed for motor height and prop pitch.

There may or may not be issues with the motor that prevent it from reaching it's peak performance, we can't tell that on our end, you will need to investigate that further.

Just putting on a higher pitch prop doesn't mean you will go faster, you need the actual power to spin it to go faster, you proved that by going from 9" to13", it did exactly what I would have expected it to do with that big of a jump, lose a significant number of RPM's.

 

[steelespike]

Right, I think I need to play with the motor height. But regardless it seems I have a problem with my OB if it can only push a 13' hull to 4700rpm/22mph. I can't imagine I'd gain 1000 rpm with heigh adjustment, and even then I would be at the OBs bare minimum.

The tach seems accurate - trolling at idle I'm getting about 900, full about 6200. But even if those numbers were way off, it doesn't explain why a 13 pitch prop is going slower than a 9 pitch for a hull this small and light.

I definitely can try an 11 pitch prop but I'd be running a lot less pitch than most folks with the same hull and still less than the stock prop.

Not sure you need to mess with the motor height.Do you have a problem with ventilation or is there an indication the motor is too low,spray or strange handling?
Tachs aren't lineal so idle rpm means nothing to overall tach performance. Tachs are always suspect when slip is unusual Yours calculates at 5%, note likely.
Very low slip may indicate the tach is reading lower than actual rpm.
The motor is also suspect when performance is in question. Speaking generically any number of things could be wrong from age or even the rebuild.
A quick compression check is a good start, also check the timing is set right and advancing as it should. check Link and sync.Does the carb open all the way.
Are the plugs the right heat range?Does the boat respond to up trim?

 

[ztrain727]

I lowered the jack plate 2.5" and didn't see any difference in RPM or speed.

I don't know of anyone with an identical setup, but I'm working as logically as I can through this. What I do know is: other BW 13' boats of this vintage get between 28-33mph with 30hp. This I have verified any different places. I also know a 30hp on a 13ft boat should not have a 9 pitch prop and should be hitting the rev limiter at closer to 2/3 throttle. This was confirmed by the Honda dealer.

As for the motor: the plugs are what the manual specs, not sure about the timing but the rebuild was done by a marine mechanic. I verified it is a 2:1 gear ratio with your technique. The carbs seem to be functioning well and all plugs are a light brown and look clean. I suppose the tach could be misreading but why would I go slower with a 13 pitch prop than a 9 unless my RPMs were way off.

I guess I'll check the compression next chance I get.

 

[ondarvr]

You have tried two props, both at the extreme ends of high low pitch, and amazingly you have hit RPM's at the extremes too, this is exactly what would be expected to happen. Now try a prop in the middle and see what the results are.

 

[steelespike]

Do you know the prop size for the various speeds. If we calculate with a 12" pitch,12% slip,2.08 ratio and 33 mph we get about 6600 rpm.
A 13" figures to about 6300 rpm.I'm assuming 12% If we assume 10% slip, a really good but reasonable, slip with a 13 we get about 6200 rpm. Of course we have to assume the motor can twist a 13 to up to 6200.
I think a further investigation into the motor may discover any problems.
Does the boat respond when you increase up trim?

 

[Sea Rider]

You can't possibly dial best engine/transom height for best combo performance if unsure tach, engine & prop works 100% at its peak performance. Does your 30 OB has bracket holes to raise/lower engine, or it's a clamp model OB ? You haven't said a thing about if you experience excessive aeration when at close tight turns on a dime while having that AV plate highly exposed. If so, you'll experience same while riding on choppy/windy water cond. Riding a combo with exposed AV plate is only good for straight flat water courses.

We experienced same situation as yours with a client on same length & boat type, that's AV plate running highly exposed, combo was slow to plane and felt sluggish while being powered with a 40 HP in excellent shape, prop included. 2 issues accomplish that slow water behaviour, boat's hull was excessively water lodged and OB not opt height seated for top combo performance. Hull took one month to be 100% dried out while boat sunbathed in plain summer heat and long sunny days.



Transom was raised 1 3/4" at marina, now water flow at speed skims right under WD plate (Yellow line) has a scary performance, achieves fast acceleration, excellent hole shot, superb tight close turns at speed and overall top performance under all water cond.



Medium pitched stock prop performed beautifully, no need going for a prop maximization for owner's particular needs. You don't need to play with trim once engine is correctly height seated and know how to balance deck properly.

If you like boating as up to know, it's your choice and entirely up to you, but if you wish dialing an excellent boating experience post lower leg pics shot sideways as in second pic to suggest ideal engine height for that combo.

Find disgusting that theoretic boaters which doesn't sell OB's, watercrafts, install new OB's on them nor performs wot water trials to dial best engine/transom heights for owner as to have a personalized boating experience likes to discredit other posters on themes he's doesn't have the slight idea about or is unable to mentally understand real world proven practice, Redial, ALL OB's delivered with stock props on them are medium pitched or close to it and that's the first prop you should test "before" opting for a prop max to suit your personal boating preferences, that's speeding, leisure, family oriented, pulling toys or even skiing.

Happy Boating

 

[steelespike]

Gee, another prime example going + 500 revs with one more pitch size while maintaining same prop diam size. If 6200 is the max wot rpm for that OB, you're right there while lightly loaded and solo boating. Yo have an engine/transom height isssue, the idiotic rule of thumb "AV plate even with hul " is killing the top performance you can get out of that 30. There's a miss match between lower leg WD plate and water flow passing at speed,

While AV is being hightly exposed prop is loosing best prop grip & thrust and if badly trimmed much worst. Will assure that at tight close turns while on plane that prop will aerate badly, right now only good for straight water courses on flat water. Personally would stay with stock prop along a engine height optimization, going one less pitch will shoot rpm by another 500 rpm, only good if boating medium to heavily loaded to compensate rpm loss with stock prop.

Post a side pic of your engine showing lower leg's both plates to have a look.

Happy Boating

Do you have to be so snotty? Just because a a procedure is different than yours doesn't mean it is wrong.
Was the setting in the picture your one and only adjustment?
Its a wonder that the boat handles normally when the foil appears to be 2" under water.
When a boater happens to select a prop incorrectly A friendly Explanation of basic prop selection and boat setup
is the way to handle things.If a motor is setup too high the tach and slip will tell.
IROT Is just plain inflammatory. Rule of thumb is again a starting point.It appears that on your example "about even with the bottom"
is too high.
I've been on here 12 years.Your attitude drives people away.

 

[GA_Boater]

Guys - Please get some new bottles of water, there must be something in the ones you're drinking from now.

We all have opinions and no opinion is more right than any other. Please be civil to one another and keep an even tone. If you find a comment rubs the wrong way, ignore it and simply state your alternate thoughts. We know it isn't always easy.

Thank you,

The Mod Team

 

[steelespike]

I think I had some of that old inflammable Hudson river water. Now I have a fresh Skaneatles highball. Pure H2O.

 

[Sea Rider]

Do you have to be so snotty? Just because a a procedure is different than yours doesn't mean it is wrong.
Was the setting in the picture your one and only adjustment?
Its a wonder that the boat handles normally when the foil appears to be 2" under water.
When a boater happens to select a prop incorrectly A friendly Explanation of basic prop selection and boat setup
is the way to handle things.If a motor is setup too high the tach and slip will tell.
IROT Is just plain inflammatory. Rule of thumb is again a starting point.It appears that on your example "about even with the bottom"
is too high.
I've been on here 12 years.Your attitude drives people away.

OK, as suggested by forum moderator let's be polite, what bothers me is that after posting lots of proven examples which no one does, which are product of plain practice even personalizing ones for forum menbers. Only 2 boaters have issues with my posts when having lots of happy boaters in UK, USA, Canada, Australia, to name some countries, boaters that have benefited reading my posts and going after them. Doesn't have to do with being snotty, it's that like sharing my boating experience and make boaters happy, that's the purpose & spirit of the forums.

Going back to the posted example :

A boat/engine combo is not a car, van, motorcycle in which you only seat, drive and have fun with. A combo is governed by different tech parameters, an innefective instalation procedure will not take all the ponies out of under powered OB's if in pursuit of achieving so. OP should test t both posters given parameters by himself and determine which one worked for him.

Besides spot on engine height, which other adjustment was there to do, playing with trim settings ? as said it's a manual trim, settting was dialed at a hole that achieved combo riding parallel to water level with well balanced deck. In this particular case, OP is not going to chop transom down, the only way to solve that highly exposed AV plate would be installing a exterior jack plate and play with different engine heights till spot on height is dialed as of the lower leg posted pic. If OB is real heavy (4 strokes) Jack plate could off-set OB weight at back transom.

To state that combo performs wonderfully with 2" under water must do so in all water conditions. OP has not said a word about having tested that combo on choppy windy straight water courses, even at close tight turns at speed, probably being a newby likes the actual behaviour of that combo, if that's ok, to each his own.

Don't have prop issues, you should always test stock props or at least the one that is currently installed when buying second hand combos, then go from there selecting other pitch props to suit your boating applications, at least we fully agree on that point.

Having a long boating life says nothing if someone keeps schooling other boaters with hints, rules that haven't been fully tested and evaluated during plain water practice by "themselves".

Did the Wright brothers flew by plain theory ? definitely not, give other posters plain practice a water evaluation, you'll be highly surprised on what can be achieved with under powered OB' as opposed to boating theory.

Happy Boating