Is the OB class doomed?

[LohringMiller]

A picture to show the motor better. It's a real outboard.

Lohring Miller

 

[Carl Van Houten]

A picture to show the motor better. It's a real outboard.

Lohring Miller
View attachment 328459

A few years ago I built two custom lower units similar to the one shown in your picture. I liked it a lot because it moved the C/G forward and helped the weight distribution of the boat but by fixing one issue it created another. Even with a perfectly balanced prop, as you increase the distance between the mounts and the propeller, you also increase the props ability to to transfer its vibration resonance from the propeller blades striking the surface back into the lower unit. this was a large prop and that made it even worse.

When you lean the engine forward and stretch the prop way behind the mounting location, the lower unit acts like a tuning fork and amplifies any vibration causing failures with fasteners and cracking things that normally never fail. It was fun but was another learning curve. If I ever make another one I will keep that in mind. I most likely made my lower (not knowing at the time) that was perfectly resonate with the RPM range we were hitting.

If you make a custom stretched and laid down lower unit and you don't have these issues you were lucky.

 

[HTV Boats]

Knowing basically nothing about a Canard I figure the best way is just build and test one. Going to be an FE OB.
(dam you Carl)

took a Zip OB kit for the tub and modified things. Any thoughts on canards welcome.
Mic

 

[Carl Van Houten]

Knowing basically nothing about a Canard I figure the best way is just build and test one. Going to be an FE OB.
(dam you Carl)
View attachment 328499View attachment 328500
took a Zip OB kit for the tub and modified things. Any thoughts on canards welcome.
Mic

Just to clear up what "****" means, it must mean Love! LOL

 

[dave roach]

Hay Mic Doe's the sponsons go on the front LOL

 

[Alan Elzer]

A few years ago I built two custom lower units similar to the one shown in your picture. I liked it a lot because it moved the C/G forward and helped the weight distribution of the boat but by fixing one issue it created another. Even with a perfectly balanced prop, as you increase the distance between the mounts and the propeller, you also increase the props ability to to transfer its vibration resonance from the propeller blades striking the surface back into the lower unit. this was a large prop and that made it even worse.

When you lean the engine forward and stretch the prop way behind the mounting location, the lower unit acts like a tuning fork and amplifies any vibration causing failures with fasteners and cracking things that normally never fail. It was fun but was another learning curve. If I ever make another one I will keep that in mind. I most likely made my lower (not knowing at the time) that was perfectly resonate with the RPM range we were hitting.

If you make a custom stretched and laid down lower unit and you don't have these issues you were lucky. View attachment 328461

A few years ago I built two custom lower units similar to the one shown in your picture. I liked it a lot because it moved the C/G forward and helped the weight distribution of the boat but by fixing one issue it created another. Even with a perfectly balanced prop, as you increase the distance between the mounts and the propeller, you also increase the props ability to to transfer its vibration resonance from the propeller blades striking the surface back into the lower unit. this was a large prop and that made it even worse.

When you lean the engine forward and stretch the prop way behind the mounting location, the lower unit acts like a tuning fork and amplifies any vibration causing failures with fasteners and cracking things that normally never fail. It was fun but was another learning curve. If I ever make another one I will keep that in mind. I most likely made my lower (not knowing at the time) that was perfectly resonate with the RPM range we were hitting.

If you make a custom stretched and laid down lower unit and you don't have these issues you were lucky. View attachment 328461

That's definitely a Nelson motor

 

[LohringMiller]

Mike ran a stock, air cooled Mac 3.5 standard rotation engine. We had no particular vibration problems. The boat has several features that Mike developed over years of testing that made his SAW boats fast and stable.

The aft main wing coupled with forward sponsons lifts most of the boat's weight out of the water and gives great longitudinal stability. Air trapped sponsons lift much of the remaining weight out of the water while creating troughs that help hold the run straight. These design features really helped on heavier gas boats. We got a 3 mph increase with air trapped sponsons on the 110 mph gas record holder. The first runs with the outboard hydro showed that the wing was too big so we took a little off the ends on the final boat.

Balancing air and water lift is critical for high speed success. Tunnels have this problem in spades. Any design has a maximum speed where the tunnel lift is too high. Hydros can get around this but it takes an understanding of the forces involved. Mike has the best feel for boat design of anyone I've worked with. It was developed over years of trying everything we could think of. The fact that no one has beat this O/B hydro record is evidence of this.

Lohring Miller

 

[Carl Van Houten]

Mike ran a stock, air cooled Mac 3.5 standard rotation engine. We had no particular vibration problems. The boat has several features that Mike developed over years of testing that made his SAW boats fast and stable.

The aft main wing coupled with forward sponsons lifts most of the boat's weight out of the water and gives great longitudinal stability. Air trapped sponsons lift much of the remaining weight out of the water while creating troughs that help hold the run straight. These design features really helped on heavier gas boats. We got a 3 mph increase with air trapped sponsons on the 110 mph gas record holder. The first runs with the outboard hydro showed that the wing was too big so we took a little off the ends on the final boat.

Balancing air and water lift is critical for high speed success. Tunnels have this problem in spades. Any design has a maximum speed where the tunnel lift is too high. Hydros can get around this but it takes an understanding of the forces involved. Mike has the best feel for boat design of anyone I've worked with. It was developed over years of trying everything we could think of. The fact that no one has beat this O/B hydro record is evidence of this.

81.5 mph?...With a custom made lower unit, custom made hydro, a high timed engine and tuned pipe? I would think that record should be in the mid 90's. In 2005 This was a good time but a round track Zippkit hydro holds the IMPBA O/B Hydro record within 5mph of this record and the 3.5cc inboard record is now 41.1 mph faster than that time. This record needs to be revisited soon because I am certain that their is much more left on the table. Where is that boat? Ya'll get it back out and give it a go.

 

[LohringMiller]

According to the IMPBA web site the IMPBA B outboard hydro record is 76.5 mph. The IMPBA B tunnel record is 68.8 mph. The motor was a stock out of the box MAC 21. At the time the 3.5 riggers were running under 100 mph, but Mark Grim pushed that NAMBA record over 100 mph in a great competition with Steve O'Donnell. Mike is now in England and has moved on to Offshore racing and fun model boats

Lohring Miller

 

[Carl Van Houten]

According to the IMPBA web site the IMPBA B outboard hydro record is 76.5 mph. The IMPBA B tunnel record is 68.8 mph. The motor was a stock out of the box MAC 21. At the time the 3.5 riggers were running under 100 mph, but Mark Grim pushed that NAMBA record over 100 mph in a great competition with Steve O'Donnell. Mike is now in England and has moved on to Offshore racing and fun model boats

Lohring Miller

I was hoping Mike would make another run. Yes, you are correct the IMPBA current record is 76.5 mph (within 5 mph of Mike record) set by David Hall. With all that has been learned and developed since 2005, someone should go after these O/B records soon.
-Carl

 

[Carl Van Houten]

I talked to Lohring the other day. We both have similar ideas. I sent him a drawing of one of my new O/B hydro designs. Maybe some of these will get out on the track this spring.

 

[LohringMiller]

Before I got Covid, I've been trying to get a 34" XT-460 to run with one of the new TenShock motors on a K&B 7,5 lower unit. The hull I'm running has won and placed at several NAMBA Nationals. It's run everything from a 3.5 mod engine to an 11 cc mod engine. I'm running the same balance point as the 11 cc version did and the weight is similar.

However, the highest speed I've radared an XT-460 was 61 mph driven by Frank Crocket in a heat race. In the distant past I held the 11 cc mod SAW record at 57 mph with an XT-460 . This boat is running over 70 mph as calculated from the motor rpm and x450 prop. The motor could easily run a bigger prop, but the boat can't handle the x450. I still can't keep the boat on the water, but the cornering is OK. I'll work on it more at the NAMBA Nationals, hopefully with some help from the NW tunnel guys.

I also have been thinking about an outboard hydro. Carl and I have been exchanging drawings. I think a big problem is the P length limit. It's starting to result in hard to control little hulls in several classes. Brian Buaas had the thought that an 8S limited class can be inexpensive with the current equipment. He is testing this on a Mutt sport 40 I loaned him.

Lohring Miller

 

[Carl Van Houten]

I just finished a 38" 8S O/B Hydro. I'll send it to the laser in a couple of days, build it and we will see what she will do.