Can't set outboard trim to exactly 90 deg - in theory, what's the ideal compromise?

[decrepit]

I know I can figure this out by taking a minute in the boat, but we're windy right now.

The boat's a SIB with lifters installed (so a constant downward force on bow, as you can't adjust these while running). Because of the transom angle, the engine will not settle at 90 degrees - I have a choice between hole 2, slightly inward, or hole 3, slightly outward.

I'f I'm looking for maximum wot speed, and taking into consideration the lifters (which would increase drag the farther out the engine goes), which of my two trim options is ideal?

The problem I can't wrap my head around is that setting the engine inward will increase drag on the bow, and setting the engine outward will increase drag on the lifters, but which drag is more detrimental to speed?

I've always set at hole 2, slightly in, as this was ideal before I installed the lifters. Since the lifters keep bow down, I'm wondering what setting the engine slightly outward would do. How would it change control, speed, etc?

Edit - This is only vaguely prop related, really an engine tilt question, sorry I'm not sure if I've posted in the right spot.

 

[H20Rat]

Pics...

By lifters, do you mean smart tabs, trim tabs, or whale tail? And the only sure answer is with a GPS. GENERALLY tilting the engine up will increase speed, but there are lots of cases that doesn't work quite so cleanly. (I'm assuming this is also on a boat without power trim, but again, pics and a little more description would be most helpful!)

 

[steelespike]

It has been my experience with manual trim that very often one notch isn't enough and the next is too much.
You can buy or make wedges that can add or subtract half a notch.Don't worry about an exact degree of trim.
Is SIB a "soft inflatable boat"?What is the horse power/speed?
The "lifters" should only be set to maintain the best trim.I'm guessing they are called lifters because they lift the stern.
If lifted too much it makes the boat plow.

 

[decrepit]

The boat's a 12' soft bottom inflatable with keel, and the lifters are the Maxi Marine "clown shoes" type. The engine's a 9.8 Nissan 2 stroke and it's manual trim. The boat clocked 15 MPH with 2 in mild chop when tested, which is typical but on the lower side. The lifters improve the ride significantly, but I did find I was plowing while loaded so trimming slightly up might work for me.

 

[decrepit]

I'm just wondering what would slow me down more - drag from the bow plowing, or drag from increased force on the lifters if engine is trimmed out. I'll get out in the water and find out soon though.

 

[Scott Danforth]

I'm just wondering what would slow me down more - drag from the bow plowing, or drag from increased force on the lifters if engine is trimmed out. I'll get out in the water and find out soon though.

or both

 

[jimmbo]

Can the trim tabs be adjusted to they are parallel to the bottom? That way they act as an extension of the hull and then your engine trim wouldn't be fighting the trim tabs.

 

[decrepit]

Yes - the tabs can be set this way, and that's how I adjusted them today. I moved to hole three on the outboard, so slightly outward - I must have been digging the bow in before, it was a hell of a lot softer of a ride today than it's ever been. Caught some crab, good day!

 

[Sea Rider]

A 9.8 HP OB is way underpowered for a 12 footer Sib, generally rated for 15 HP. What you can do to get the max out of that combo is inflate Sib to its max 3.5 PSI on all air chambers, keel included. OB should ride at 90 deg when combo is on perfect plane parallel to water level. Better is to be slightly overtrimmed + one hole, can balance deck weight bit forward to compensate.

Other improvement, get a induction Hardline tach and tach that 9.8 with just you alone and lightly loaded, if needed, move forward to plane faster , check wot rpm, see if around 6000 rpm, depending on the wot run can improve hole shot maximizing a less prop pitch for combo to plane faster.

Happy Boating

 

[decrepit]

I use a high pressure pump and fill the tubes to 3.5 psi, 5 in the keel. I think I am hitting the max the 9.8 will get (the trim change didn't change wot speed but a lot less banging into waves).

I previously posted, debating upgrading the engine, but since we like the speed with 2, 90 percent of the time only ride with 2, and the motor has been beautifully reliable, was "like new" when I bought it, and is the lightest 9.8 I will ever find (less than 60 pounds), I've decided to keep it as it is. I picked the motor up while holding a 20 pound dumbell to get an idea of how carrying a larger motor would feel, and decided I'd hate it. Our guests will have to live with displacement speeds if they want to visit the islands

Searider, I'm going to hook up a tach to it to find out rpms. The only reason I'd put money into a new prop is if the boat could plane with 3 with one. Is that possible, or something only a larger engine would achieve?

 

[Sea Rider]

If Sib is inflated to its max working presssure, lets work out the second alternative..

Need to know current max wot rpm with you alone while lightly loaded. You'll be surprised what a prop maximization can do to your beloved 9.8 and current 2 passengers as well. I'm boating using my beloved 18 HP on a larger Rib intended for a larger OB with just a prop maximization, As a test, tach OB with you alone, record data, then add second boater and record data, which is the rpm difference between both trials. Maximizing a less prop pitch can add higher rpm to the equation but to a limited extent along carrying max number of passengers. Probably a third boater will be too much weight, but you'll never know till a water test is done with maximized prop.

Other issue, on choppy sea water cond just throttle enough to pass fast displacement speed onto minimum plane, bumpy rides are awful for passengers, are the ones that will jump more and hit thermselves as opposed to driver. The overall ideal is to provide a comfy ride for your passengers. Don't buy cheapo inductions tachs, get a Hardile hour tach at Amazon and select 2P setting, (2 spark per revolution) post wot results.

Happy Boating

 

[decrepit]

To follow up on this - I haven't done anything with props, but adjusting the engine trim slightly upward increased the speed from 10 km/h with 3 people to 18 km/h. A slight adjustment to the lifters (angle adjusted upward) probably contributed to this too. There is no different in top speed with 2 people (max was and likely always will be 26 km/h).

Almost double the speed with 3 on board is enough to keep me from complaining.

 

[Sea Rider]

Decrepit,

For OB to play on the safe side, install an induction tach to check max wot revs with 1-2-3 boaters trimmed as last outing.. The more boaters you add the more wot rpm that will lose. Should not lug that 9.8 with 3 up, it's bad music.

Happy Boating