Mounting twin outboards to inflatable

[freebird164]

I have a 15' Saturn SD470 powered by a single 20hp short shaft Suzuki motor. The boat is rated for up to a 40hp. I am wanting to mount twin 20's (for redundancy when boating offshore). The motors each weigh approx. 100 lbs., so I don't think the weight will be an issue. The problem is that the transom is designed for a single motor setup. The transom is scalloped and slopping so there is not currently enough space to mount twins. The options I have considered are -
1. Cutting the necessary space in the transom to accommodate the second motor and re-enforcing the transom with bands or plates to make up for the loss of structural support.
2. Building up the transom with an extension plate and going to twin long shaft motors. (Very expensive so not my favorite option).
3. Having a bracket built for the motors and attaching it to the existing transom. (This is the option that I like the most).
Does anyone have any suggestions regarding these or other options?

 

[Papa-t]

Or if you have the money sell the 20 and buy a 40.

 

[matt167]

Double the weight sounds like a horrible idea

 

[Sea Rider]

Welcome to IBoats,

What's the sole purpose of such cool idea asside throwiiiiig money down the deep blue, although 20 + 20 adds up being 40, won't double the max speed, just slight more top end increse. Will only achieve handling more load, passengers more efficiently, better overall thrust at a much higher fuel consumption and the extra cost of the second motor.

Better go cheap, buy an induction tach, would ecommend this nice one : https://www.hardlineproducts.com/product/hourtach-meter/ check Iboats, Amazon for them, install it following instructions to the letter, go for a wot run as loaded, check achieved wot rpm numbers and compare them against the min-max wot rpm range factory stated for said Suzy motor, will surely need to go to less pitch props to bump the wot rpm to around middle to max rpm range. If will be using that large Sib lightly to medium loaded go for a 20 HP well propped, if higly loaded go for a 40 well propped as well...

If that Sib is rated for a short shaft motor don' t need to go for long shaft one, the lower leg will achieve an excessive back water or over transom splash in which you'll be able to refresh yourself there LOL!!

Happy Boating​

 

[freebird164]

Welcome to IBoats,

What's the sole purpose of such cool idea asside throwiiiiig money down the deep blue, although 20 + 20 adds up being 40, won't double the max speed, just slight more top end increse. Will only achieve handling more load, passengers more efficiently, better overall thrust at a much higher fuel consumption and the extra cost of the second motor.

Better go cheap, buy an induction tach, would ecommend this nice one : https://www.hardlineproducts.com/product/hourtach-meter/ check Iboats, Amazon for them, install it following instructions to the letter, go for a wot run as loaded, check achieved wot rpm numbers and compare them against the min-max wot rpm range factory stated for said Suzy motor, will surely need to go to less pitch props to bump the wot rpm to around middle to max rpm range. If will be using that large Sib lightly to medium loaded go for a 20 HP well propped, if higly loaded go for a 40 well propped as well...

If that Sib is rated for a short shaft motor don' t need to go for long shaft one, the lower leg will achieve an excessive back water or over transom splash in which you'll be able to refresh yourself there LOL!!

Happy Boating​

Thank you all for the responses. The sole purpose behind this project is redundancy in case of a prop strike or mechanical malfunction. Realizing the cost of an additional motor that HOPEFULLY will never be needed, I am all about self reliance. We will be several miles off-shore. We have radio and GPS, but I'm not one who tends to have a lot of confidence in technology when trouble arises. Regarding the weight - the twin 20's would be slightly less weight than a single 40 (and again, looking for redundancy).
The idea of using long shaft twins was only if I were to build the transom up to the upper height of the existing transom instead of trying to mount in the notch. That would increase my motor height by about 4 - 5 inches.

 

[Scott Danforth]

adding 2 20hp motors does a few things:

• it overly complicates your boat
• it adds unnecessary weight.
• not to mention that you will run into the tubes at certain angles of tilt and steering.

having two motors does double the chances of something going wrong.

I would just buy one of these, put it in a ditch bag in the bow with a jerry can of fuel https://www.amazon.com/CDZHLTG-Outboard-Cylinder-Inflatable-Sailboats/dp/B093L76RCX/ref and save yourself 80# to have to haul around

that way if you have a problem, you clamp it on, add fuel, then and get back to shore.

 

[freebird164]

adding 2 20hp motors does a few things:

• it overly complicates your boat
• it adds unnecessary weight.
• not to mention that you will run into the tubes at certain angles of tilt and steering.

having two motors does double the chances of something going wrong.

I would just buy one of these, put it in a ditch bag in the bow with a jerry can of fuel https://www.amazon.com/CDZHLTG-Outboard-Cylinder-Inflatable-Sailboats/dp/B093L76RCX/ref and save yourself 80# to have to haul around

that way if you have a problem, you clamp it on, add fuel, then and get back to shore.

Thank you for your response. We use something similar to this (although it is electric and has only 40 lbs of thrust) when we are on inland waters. Works really well unless we are fighting a head wind. Could something like what you're suggesting reasonably be expected to get us back to shore in off-shore conditions? We had looked at using a 6hp in this manner but I was afraid it wouldn't have enough push to fight a moderate current.

 

[Sea Rider]

Continuing with said fun Project : So you want to duplicate this dual motor Sib installation...

For minimum transom work and extra expense will need to modify your current transom to achieve a straight transom (full red line extent) to hold and comfy sit 2 motors next to each other. What's the current transom height (yellow line) as seen ?​

Tech issues : How will you solve the issue about tiller steering and throttling 2 motors at same time ? For that will need to add a center console and a single remote box that can gear and throttle both motors and a remote wheel with a single cable linked to both to drive them while comfy seated sipping cold beers...

Better do nothing as this exercise has been already duplicated with the sole intention that can be done along being eye catching and cool but for no practical purpose at all. If plan using any of both distanced motors from each other as an emergency kicker will need to fully raise one and a single motor powering sideways a boat at high throttle will tend to make the Sib to veer to a side, will need to constantly be steering to correct that issue. Now imagine the less available deck space which it's a must count 2 fuel tanks will dial....

How loaded do you plan going boating with which it's key to know ? That's a very large Sib with extra hull drag. If haven't purchased any motor yet go for at short shaft 25-30 HP motor and sit it right at middle transom and buy an extra motor say a 9.8 to be used as an emergency aux, Can store it laying at front deck. Ideal would be for both to go for a prop maximization to run towards their max wot rpm range factory stated as loaded to get the max water fun out of both specially in Off Shore windy, strong current scenarios...

Happy Boating​

 

[Scott Danforth]

Thank you for your response. We use something similar to this (although it is electric and has only 40 lbs of thrust) when we are on inland waters. Works really well unless we are fighting a head wind. Could something like what you're suggesting reasonably be expected to get us back to shore in off-shore conditions? We had looked at using a 6hp in this manner but I was afraid it wouldn't have enough push to fight a moderate current.

a buddy of mine moved his 37 foot sailboat across Tampa bay in a head-wind using the 2hp outboard on his dingy. it took a few hours to go 8 miles, however he did it.

 

[freebird164]

Thank you all for your input. I have taken all of the comments into consideration and have decided that a simple emergency motor would be my best option. One last question - We are going to be operating in the Florida, Alabama and Texas Gulf areas. A VERY dynamic area and often very unpredictable conditions. We will be between 9 and 10 miles offshore. At maximum, our team consists of 4 equipped scuba divers and a boat tender. Balancing weight vs. power, would a 6hp emergency motor be reasonable?

 

[roscoe]

If you truly want redundancy for your crew of 5, get a second boat.
2 motors, separate fuel cells, extra carrying capacity/space.
Easily tow one in if it becomes disabled.

 

[Sea Rider]

A 15 Ft Sib is an excellent carrier platform for diving purposes, will handle 4 divers and diving gear which equates having 5 divers and a tender man on board, to work well in extreme variable weather conditions will be better to count with a 30 HP short shaft motor.

Portable motors factory delivered with a prop are medium pitched. If you already count with a 20 HP motor and don't plan going to a higher HP motor any time soon will be mandatory to wot test current prop with 5 souls, full diving gear, fuel tanks, beer coolers and check if the motor revs to at least to its middle wot rpm range when at full throttle. Being heavyly loaded will surely lug the motor out badly when full throttle is in demand. Will need to install an induction tach to check that out yes or yes...

For an aux motor personally wouldn't want to count with a 6 HP motor, your inflatable is very large, wide, runing it highly loaded will add extra hull drag, better go for a 9.8 HP aux well propped. In case the main fails and the water cond gets windy will take you back to Terra Firme at a slow safe pace. Been there myself in + 50 KM and higher awful winds, that's why the recommendation...

Cheaper is buying a nice aux for the main and avoid buying a second equivalent boat for that purpose.

Happy Boating​

 

[freebird164]

A 15 Ft Sib is an excellent carrier platform for diving purposes, will handle 4 divers and diving gear which equates having 5 divers and a tender man on board, to work well in extreme variable weather conditions will be better to count with a 30 HP short shaft motor.

Portable motors factory delivered with a prop are medium pitched. If you already count with a 20 HP motor and don't plan going to a higher HP motor any time soon will be mandatory to wot test current prop with 5 souls, full diving gear, fuel tanks, beer coolers and check if the motor revs to at least to its middle wot rpm range when at full throttle. Being heavyly loaded will surely lug the motor out badly when full throttle is in demand. Will need to install an induction tach to check that out yes or yes...

For an aux motor personally wouldn't want to count with a 6 HP motor, your inflatable is very large, wide, runing it highly loaded will add extra hull drag, better go for a 9.8 HP aux well propped. In case the main fails and the water cond gets windy will take you back to Terra Firme at a slow safe pace. Been there myself in + 50 KM and higher awful winds, that's why the recommendation...

Cheaper is buying a nice aux for the main and avoid buying a second equivalent boat for that purpose.

Happy Boating​

Thank you Sea Rider! You have been a wealth of information!

 

[Sea Rider]

You're welcome FB 64,

Out of curiosity report which aux motor brand and HP ended buying. Would suggest to buy a nice pressure gauge for that large inflatable carrier, pump all the air chambers and keel to their max recommended working pressure to ensure an overal rigid structure specially when boating heavy loaded. Any other matter you want to know about, just ask...

Happy Boating​

 

[Bt Doctur]

you must have a serious death with to operate a small inflatable "off shore"
Just where do you mount a vhf radio and ant?

 

[Sea Rider]

you must have a serious death with to operate a small inflatable "off shore" Just where do you mount a vhf radio and ant?

As per the OP's post 9 to 10 miles off-shore it's nothing compared to the usual 20 to 30 miles off shore boating to far away islets I usually do with a 15 footer Rib powered with just a 18 HP motor. Just check the weather conditions prior outing and carry a powerfull hand held WHF radio as a safe precaution. WHF radio waves travels horizontally and at such small unobstructed distance from dry land don't need any extra antena and surely will see many passerby boats if serious help is required...

Happy Boating​