LohringMiller
Well-Known Member
- Joined
- Jan 25, 2017
- Messages
- 928
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.
Just to clear up what "****" means, it must mean Love! LOLKnowing 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
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 motorA 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
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.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.
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.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
Enter your email address to join: