Off centered motor on a single V Hull

[LeonardG]

Hi,

Bought an Exocet 23 (piranha hull with phantom deck) and I put a 300 R, 1.75 TM, on it (not an SM, no hydraulic jackplate on it).
The boat have no pad, no trim tabs and I don't want to put trim tabs ; It's a deep V.
It is 2000 lbs.

The motor is clearly off centered, about 2" (you can see it on the pic).

Discuting with the builder, he told me he never centered the motor to compensate the torque (he was racing with the boat and have really good result) ;
But I have serious porpoise issue when I pass 70 mph. My aim is not to race, but to top speed.

Do you have any idea about what it could do to centered the motor ? (I will centered it to try anyway, less drag and it may stop porpoise)

Did you ever had those porpoise issue ? Like crab walk with of centered motor on a single V ?

All experience's return are welcome.

Thank you.

[LakeFever]

That’s odd I’m no racer but I’ve never seen anyone offset a single motor like that before.

I’d drill new mounting holes and repair the existing holes. Especially being two inches out the repair would be pretty easy I would think. I’d recommend epoxy or vinylester resin and make a cup of milled fiber and cabosil paste. I’d tape some parchment paper over the inside face of the holes and packing from the outside in use the milled fiber paste and hydraulically fill those holes very, very well. No voids. Once cured depending on how I felt about that I’d either refinish as is or I’d lay some 1708 and csm over the holes and refinish to suit although I’d likely want to try and just touch up the gel without using mat first as this would be a lot quicker.

Then drill new holes in the center and go boating

[OnPad]

A offset stinger, or strut is common in rc boat racing. This application aims the drive at a slight angle, in order to reduce prop walk, or crab on very fast inboard boats. The prop torque is usually delt with by loading the boat heavier to one side. I've never seen it done on a outboard, as they are slower, and have thrust vectoring. That being said, the builder might have known what he was doing, If the boat is balanced/level when running fast. Alot of times porpoise is a effect of boat design, too much trim, or too much weight at the transom end of the boat. Hard to know if remounting the engine to center will help.

[LakeFever]

Agree regarding porpoising some hulls just can’t help themselves from it. Most performance v’s do it some worse than others. All my streams have porpoised usually around 35-50mph or so then they settle down. Anyways this thread is moving north of my pay grade on this topic im sure others will chime in. Sometimes it takes a few days not everyone is on the site everyday

[LeonardG]

The hull should not porpoise, piranha 21 carrera (the hull base) are not known for that (I think I am right on it).

I only have the motor on the back of the boat, battery and hydraulic direction's pump are 5ft in front of the front seats and tank is in the bow, centered just behing front seats (when I swap the battery from the back to the front, less porpoise)

I only can gain 22lbs on the back changing the setting when centering de motor (transom was cutted for a 15", I will modifie the plate and gain 22lb), all I can do.

If it continue to porpoise, I will change the battery place (to much on the "nose" will amplify the porpoise I think, near the frond sit in the bow will be better), but I go step by step.

[David - WI]

How much horsepower did the previous owner have on it? Usually if it's starting to porpoise you need to add more power or less trim at that speed.

[KIRCHNER]

i have 2 skaters, 1 full tunnel, 1 has center pod. cant stop porpoise. just call me bronco billy. not happy, i have a 15' 9'' magnolia cat 1962 w/ promax, runs flat, any speed, does 80. go figure...

[LeonardG]

Previous owners had 2.5, 300X and 300 XS on it. He told me the boat don't like the weight in the back (but I can't reduice my 300R weight, I have a CMS, only 12 lb more than an optimax on the paper).

The boat is rated for 300 HP (and 250 R and 300 R are same weight, so I went for a 300).

Concerning purpose on skater, you trim to much, I have a 21.
Porpoise is hard to stop but when you place the skater "on the rail", with less trim, you don't have porpoise and can go top speed easily. Cats hates trim to much. The speed come when you make the tunnel.

[kelvin523]

This looks a bit like my 24' Superboat (2000lbs, 7' beam, 300r), honestly never heard of crab walk is that like chine walk? I cant say I've ever had proposing over 55mph. We've worked on the setup a bit, with still more to do. Engine height with the 300r is critical; I assume you have the bullet set at minimum to the bottom of the V', higher is even better. The bullet even a little in the water caused high speed handling issues. Currently running a hefty set back as high as the motor can go with a 25p promax spinning 6100 hitting 74mph. still have 20psi of water; so working on raising it even higher to pull a few more mph.

As for the off center; hey, that's a first I've seen. I'm aware of other ways to accomplish a similar counter torque without putting the motor off center; the 300R skeg is also modified for this reason.

I don't see any relation to this issue and cat's in ground effect, hope helps.

[OnPad]

When the op said crab walk, I figured that he ment prop walk. Maybe something was lost in translation across the Atlantic?

The only benefit I see in the offset outboard engine with a seating arrangement like shown. It would pick up cleaner, and more water to the prop.

Although I don't own a full size vee hull, I do have a pad v, and a mod v. These two boats don't have enough prop torque to counteract the weight of a single offset driver. Ballast is used on the passenger side in order to get these boat's to run level, if running solo. In my mind a true v would be even more sensitive in this regard.

I do have a v hull rc model that has trim tabs. Maybe that's a viable option to get some porpoise out?

[LeonardG]

What do you mean by bullet ? Tip of the shell ? (I'am with a torquemaster on it, as high as I can).

I don't need this counter torque, when you trim, the boat is ok and go straight.

Only porpoise after 65-70 pmh.

Anyway the motor is off to recenter it.

[LeonardG]

When I say crab walk, that mean the porpoise is strange, you feel the motor push, then you feel the V bottom making the boat have this porpoise with the nose going to the left (and you have to constantly correct it). Clearly not a chine walk but a strange porpoise.

Like if the double drag (the V and the gearcase), have an effet, when it porpoise. I don't know if you understand what I say.

[kelvin523]

Got yah, your running a TM; sorry missed that in OP, we are running a SM. Yes; bullet is the tip of "the shell". My thoughts are for me, porpoise is up and down; but i understand your description.

Does the back of the boat feel light at speed; like it wants to kick out and come around? Again, we're running a SM surface drive; but when we had the motor too low in the water, the drag on the SM gearcase had similar handling characteristics to what you describe.

I can't speak to the TM; but I really think what your doing by centering the motor is the correct call. Off center creates a whole bunch of unknown dynamics for me (eg: would you match the bullet above the V bottom to where the bottom of the boat is...etc.)

I don't know if height characteristic for the SM is the same for the TM; As mentioned we run the front of the bullet (nose cone) dead even to the bottom of the V'; and working on raising it up a few more inches.

hope helpful; look forward to update once motor is centered.

[OnPad]

I understand what you're saying very well.
Let us know how it runs after you center it.

[LeonardG]

@kelvin523

SM and TM are not the same, on TM you have to place bullet down the hull (because water come in front of the bullet and by the side like a 5.44 gearcase but lower on the side).

I got a SM on my skater with an hydraulic jackplate, but i use a TM for my V (no pad under the hull) (diameter of the "out" of the gearcase is 125mm (for a TM) instead of 136mm (for the SM) (then you have less drag on a SM as I don't use an hydraulic jackplate, you can also turn with more prop, less than 15 diameter). My best is promax prop at the moment. 5 blades give to much torque, the boat is hard to drive (I tryed the CNC of my skater, it was undrivable).

As i don't use any jackplate on it, and only have the trim the bullet is approximaly 1 inch under the hull (to be sure he have the water needed). When I trim to much, to prop lose the hook and I start losing mph (So I will probably centered it and place it one hole less to try).

For sure if I had a hydraulic jackplate, I would have put a SM. But I already have my skater 21 for this kind of things (top speed).

When you say "Off center creates a whole bunch of unknown dynamics for me", I really think so. I will place the bullet 1" under the V to test first. I don't think V are good with surface prop if you don't have jackplate and SM.

I will give a forward when the motor will be centered, probably in 2 weeks.

(and sorry for my english, not my maternal tongue).