ABC Sib-Rib Installation Guidelines.-

[Sea Rider]

A Tohatsu Especial.-

Decided to write & post this handy ABC Sib-Rib Installation Guidelines so new and old boaters use it as a reference from where to start or re start if totally not satisfied with their current sib/rib engine set up general performance. To avoid forum posters posting different ideas, myths or getting out of the central theme by other useless comparisons, these guidelines will solely and exclusively shall be applied to all OB’s brands up to max 50 HP, tiller driven, with mechanical trim, tilt and handle clamp screw. Although full concepts are quite applicable to all outboard boats in general.-

How boating passion began : Started boating way back in 96 when first bought a second hand mint condition OMC made by Zodiac 305 Mt sib along a 2 strokes Evinrude 9.9 HP engine that came as a package. Next day went boating with a friend, our first time inexperienced sib assembly was done easily fast, the nice issue was that sib came along with a Zodiac yellow round pressure gauge, push type, fixed pointing needle, internal rotating dial with well-defined psi color segments.-

After sib’s inflation completion to factory recommended working psi with delivered gauge, was immediately launched to sea, engine was secured to transom, re inflation to top PSI was made after some minutes rest on water for overall pressure to stabilize; at that time didn’t know a thing about engine trim & tilt nor what was all about. So the theory about their working principles had to be checked along under way. At that time there was no Iboats and other nice online boating forums to ask, nor commercial digital cameras to post nice instant pics, nor Youtube to post videos. internet still was very incipient.

Went for a short spin testing combo at idle, medium throttle to test sib’s response and water behavior, once confident went full throttle, soon realized it was not fun going that way but rather straight into complete frustration due to trim being locked at first hole out transom and overall sib weight badly distributed. As a teenager, when going on cars at speed, liked to pull my hand out the window playing being a wing, rotating my wrist up and down and checking the wind effect on my extended hand. Soon realized that by maintaining my hand parallel to the road gave the best cutting effect with the least wind drag.-

Having experienced this effect before on air, now on water, trimmed engine perpendicular to water level and move my boating mate up front. Next I remembered after going full throttle was achieving a fast plane and running at top engine speed with a big smile from ear to ear. On our first outing didn’t experience any engine/transom height issues nor water splashes which didn’t know about due to being both, well factory matched OMC products. -

After a while sold the OMC 305 but stayed with the 9.9 which was shortly upgraded to a 15 HP, bought a brand new Quicksilver 330 sib. Same as with previous Zodiac 305 experienced no issues at all, well weight balanced and trimmed combo planed very fast with excellent top speed.

Years after was offered the dealership of a brand new European/Asian sib/rib manufacturing facility along Tohatsu sub dealership, that seemed a perfect product marriage, but not, perfect time when all my boating bummers started. As there were two new kinds on the block, had to get rid of the 305/15 HP combo package ASAP, being in mint condition was sold fast.-

As a personal starter ordered a 360 alum panel sib with a 2 strokes Tohatsu 18 HP, seconds later on its maiden voyage at just medium throttle experimented excessive water splashes over transom. Gee, thought being just a matter of trimming engine properly; played with all possible trim settings with no avail whatsoever taking into consideration that sib was previously deck weight balanced and OB perfectly trimmed.-

This horrible and frustating boating experience taught me that in order to achieve best boat plane and engine top performance you need to have 2 issues very clearly defined and understood which are : Sib/Rib Installation, Transom Height and lately discovered OB’s Brand Match.-

Sib/Rib : All have different lengths, hull shapes, widths & tube diameters & transom heights. Inflatable boats manufacturer doesn’t care which brand engine will be powering and living on your transom unless you request a special transom height build that perfectly matches your current engine brand. All our sib-ribs models are now factory transom height matched to use Tohatsu engines without flaws.-

Outboards : Although most brands have S, L, XL sizes, all lower legs have small physical design visible differences. OB’s manufacturer doesn’t care which sib rib brand will be powered by their engines. It’s a kind of love/match, miss match/hate sib/rib//engine/transom issue.-

In order to have these boating guidelines very clear to better understand their concepts, will be divided into 5 main topics which are : (1) Boat Balance, (2) OB’s LU Tech Issues (3) Engine Brand Transom Match (4) Trim Angle (5) Sib/Rib Engine Optimization,-

Note : For best effective test, you need to have a healthy prop & max engine HP rated for that size sib/rib, worst scenario 70/75% of the max rated HP. Don’t pretend to optimum test with lower HP OB’s or unhealthy engine & prop.-

Glossary Terms : NFC (Natural Floating Condition ) BB (Boat Balance) WF (Water Flow) WDP ( Water Deflector Plate) AVP (Anti Ventilation Plate) WAE (Water Attack Edge) OTHM (Optimum Transom Height Match).-

Sib/Rib Balance & trim : Float sib/rib alone completely empty with fuel tank the furthest will go towards bow (1) while resting at Natural Floating Condition (NFC) mount engine on middle transom and manual trim engine to remain perpendicular at (90?) and AV plate parallel to water level. All sib/ribs don’t have same transom built in angle, thrust rod position could vary accordingly in between sib/rib models.-

When correctly trimmed, the brand logo on cowl will remains parallel to water level (2) Once trim angle has been perfectly set, must remain untouched at that position at all boating water conditions and load, remember it’s a fixed manual trim OB. Tight engine clamps for rear of swivel bracket to remain parallel and resting nicely against transom’s rear plate. Some ribs have a upper transom edge that needs to be filled down for OB to remain at 90? angle and sviwel bracket at perfect aligment & rest against rear plate.-

Most boaters takes for granted that just needs to sit engine on middle transom and go boating soon afterwards, but sometimes that’s not so. If sib/ribs are correctly inflated, trimmed & well deck weight balanced and on first outing you have matched perfect engine/transom height achieving non water splashes outwards or backwards whatsoever, congrats, you’re a lucky boater.-

Water Test : Before launching, with gauge inflate sib/rib to 3.0 psi, once on water and after some minutes rest for overall tubes and keel pressure to stabilize, check pressure again, re inflate to same psi if needed (3) If having an air deck sib inflate to at least 10-12 psi to achieve good floor rigidity. Rib’s tubes needs less psi for good overall performance as ribs rides on its hull not on its tubes.-

On flat calm glassy, no wind waters, sit boater mate up front towards bow to maintain sib/rib NFP. Tiller driver & spotter must sit inside deck next to transom. Full throttle, check if achieving transom water splashes to know at which lower leg's height is being produced, and if in need to optimize transom's height accordingly. Sorry folks, the only way to state of the art check water splashes issues is pulling driver/spotter head over transom (4). For boating perfection, it’s not I believe, I assume have a perfect match, I'll ask on the forums. it’s one time visual check and soon afterwards on land correction if applicable for boating perfection.-

When achieving general water splashes issues frustrated boaters usually starts to play with engine trim, a counterproductive wrong way to go. If you were correctly trimmed and weight balanced, you definitely have an engine/transom height issue, not a trim issue.-

 

[Sea Rider]

Related Topic : Engine's Clamp Pads.-

Related Topic : Engine's Clamp Pads.-

Most boaters just ties a rope from engine to transom, just in case it falls into sea, lake, river, some even don’t at all. Engine usually falls off due to clamps being released from their original locking position after initial installation, besides nobody checks engine clamp tightness for re adjustment when boating for long hours at speed or on choppy sea conditions, everybody assumes clamps will lock tight engine’s swivel bracket against transom.-

That’s not so, bear in mind that although clamps could be adjusted tight against metal plate with engine vibration, torque, water bumping at speed, clamps will end sliding upwards and engine rising to port side with the possibility of engine fall & unwanted submersion with engine working assuming it was tied to transom, If not expect a total engine loss if sea is quite deep.-

The best way to achieve engine sitting still on transom while boating on all water conditions is to install a round or square pvc, hypalon, rubber pad, between metal plate and clamps (1) Adjust tight, match both clamp’s eyes to face each other, tie with short rope.-

During engine wot trials, pic (2) engine raised (red line) more than 1”. Ooops, forgot to place clamp pads as always do. If your sib/rib transom counts with an inner metal plate, you could trim and glue a wide & long appropriate patch size to match plate’s interior contour (3)

 

[Sea Rider]

ABC Sib-Rib Installation Guidelines.-

(2) OB’s LU Tech Issues : All lower legs are not made same all have upper water deflector plates and lower AV plates although tail designs varies from manufacturer to manufacturer. If you check any engine brand LU, you’ll notice that lower rear middle leg could have (1) round border (X) or (2) square border (X) including a lower (X) water attack edge (WAE) Water splashes types :

Out transom splash (3) is usually produced when Water Flow (WF) hits round portion of leg at speed, water swirls and climbs through front attack edge and hits lower WDP deflecting water outwards onto water wake. Noticeable when engine sits higher on transom. Boater can live with it and go boating with this small bummer.-

Over transom splash, is usually produced when WF at speed hits leg’s round or square upper edge located above WDP. When sib/rib is under way at medium to full wot, produces horrible back transom splashes along excessive tail drag & deck flooding. Noticeable when engine sits lower on transom than needed. Square edges produces worst over splashes than rounded ones.-

Boater can’t go boating at all throttle ranges with this huge bummer, just at slow non planning boring displacement speed.- A transom raise optimization is mandatory by means of wooden shims, rising fiber glass transoms, installing engine mounts, etc.-

When engine sits at an optimum transom height match (OTHM) (4) won’t see this water diversion as the small water deflector plate will smooth and flat it out against passing water flow, under this condition a near flat middle wake will also be achieved.-

If having out or over transom unwanted water splashes is entirely up to boater if confy riding under or over trimmed and willing chopping down, rising transom height accordingly. Modifying transoms is time demanding, costly if done at boat's shops, along precious wasted time under trail & error until correct engine height is matched.-

Before venturing testing sib/rib/engine differently, you need to determine if chosen engine brand opt seats on sib/rib transom after following all to the letter previous starting recommendations.-

 

[Sea Rider]

ABC Sib-Rib Installation Guidelines.-

(3) Engine/Transom Height Match : Tohatsu, Merc, Nissan are natural born over transom water splashers compared to other brands like Yamaha & Suzuki mostly due to having a built upper square edge just above water deflector plate in comparison to round edges found in other OB's brands.

The usual written guidelines found on owners manuals & forum talk recommendatiosn is having AV Plate 1-2” lower with respect to boat’s bottom, personally don’t care which is the final AV height given when engine is finally seated onto transom. Sometimes plain written guidelines are opposed by plain practice. There are too many tech variables to have into account like : hull shape, length, width, tube's diam, transom height all which can vary from sib to sib, rib to rib as all are not factory made same. From experience, a given engine will not work optimum on different sib/rib brand model sizes.-

Don’t go by the owner’s manual, go by what have found during the practice course and learning curve of my boating experience installing different sib-rib and OB’s brands, including extensive sea trial & error testing and consequently afterwards corrections.-

Once you have the sweet height spot (SHS) correctly dialed in, will produce excellent hole shot, immaculate close turns at speed, better engine’s top rpm & prop thrust with optimum sib/rib performance under all water conditions.- Had a great time water testing same Tohatsu engine sitting at 90? on different transom heights and this is what was discovered while visually checking at wot.-

 

[Sea Rider]

ABC Sib-Rib Installation Guidelines.-

4-Trim Angle : A prop works as a pull/push device, that’s why is less efficient when at reverse, when prop runs vertical to water flow, the blade/water angle of attack formed will produce the best optimum thrust when throttling enough to maintain a sib-rib on plane.-

Portable OB’s top rpm & prop thrust are achieved when engine sits at 90? on transom, with properly balanced boat, prop revving vertical to water flow and AV plate riding parallel to water level assuming water flow is passing slightly under water deflector plate. Running OB’s with under or over trimmed will cause inevitable AV drag, thrust loss plainly visible if experimenting low or high rear wake when on plane.-

Tohatsu's Boating Perfection Related Links :

Tohatsu 20hp w/ 3/4" shim and proper trim - YouTube

Silverado 40S & Tohatsu 18 - YouTube

 

[fbpooler]

Re: ABC Sib-Rib Installation Guidelines.-

Very good information.

 

[Sea Rider]

ABC Sib-Rib Installation Guidelines.-

5-Sib/Rib Engine Optimization : Once engine is working at optimum height, you could play with engine prop to deliver top rpm to optimize your intended type of boating. Will need to install an induction Tiny Tach or Hardline tach, go for a wot spin as usually loaded, check achieved max wot rpm and go from there selecting a +,- pitch to have engine rpm revving inside safe min-max factory stated working parameters.-

OB’s placed on light sib/ribs carrying light loads with factory standard props usually delivers max stated wot rpm only after engine has been correctly height seated and properly trimmed. When testing underpowered OB’s for a particular sib/rib length undergoing struggling to plane issues, you’ll need to install a doel fin, trim tabs, lifters, etc. Once on plane at speed check lower leg height. Optimize if needed, if doing so, your combo will plane much faster on next outing.-

 

[Sea Rider]

Re: ABC Sib-Rib Installation Guidelines.-

6-Tohatsu’s Quick Installation Tips :

1-Inflate sib/rib evenly with gauge, at least 3.0 psi for tubes and keel, 10-12 psi for air deck if applicable. Re inflate if needed after a 5 minutes rest while on water.

2-Install engine at middle transom, secure with engine clamp pads adjusted tight.-

3-Trim engine to 90? angle once sib/rib is floating.-

4-Move gas tank & boating mate up front bow area.-

5-Full throttle, check water flow height condition.-

6-If experimenting slight out transom water splashes, live with it.-

7-If experimenting over transom water splashes, rise engine accordingly.-

Special case : Some low overboard rescue ribs, surfer transports, are being factory delivered with just 18” transom heights :

-If using a short shaft 15” OB will experiment slight back transom water splash, slight rpm and prop thrust loss, prop rides very near water line, good performer on straight lines courses on flat sea conditions, worst performer on choppy water conditions, top AV plate aeration on close turns at speed. In this particular case a transom chop down (1) is mandatory.-

-If using a long shaft 20” OB will experiment excessive LU drag, out transom water splash and worst close turns at speed, when boating at all water conditions. Will need to raise transom (2) install a stainless steel 2” engine mount to raise engine accordingly or add a second water deflector plate (3) above lower one to divert back splash against water flow and out to wake. Entirely Up to your own decision.-

 

[Sea Rider]

ABC Sib-Rib Installation Guidelines.-

7-Anti Ventilation Plate Vs Water Deflector PlateTest : There’s a lot of forum talk about AV plate should skim lightly water flow at speed for optimum performance, sorry, this myth has proven totally inaccurate. Tested same healthy engine at both lower leg heights extremes, that is AV and DF plate on a rowing track with flat glassy no wind water conditions, these are the results with a Tohatsu engine.-

Pic-1 For the test just upper small water deflector plate and lower anti ventilation plate was used (yellow portion) as we all know that if water flow hits above deflector plate will produce water splashes over or out transom.

(1) Water flow passing slightly under upper deflector plate, gives best overall water performance at all water conditions weather flat or choppy and immaculate close turns at full wot-

(2) Water flow passing slightly above anti ventilation plate performs well on straight flat courses, worst on choppy waters and at close turns at speed, excessive prop aireation, opt thrust loss; thumbs down.-

Pic-2 With water flow passing slightly under deflector plate achieved + 1 knot and 200 revs more than passing slightly above AV plate. Having more adequate water to work with prop grips much better than spinning and gripping on less level of water.-

If a healthy portable engine badly height seated loses 1+ knot and 200 revs, imagine how much speed, revs a bigger engine will lose and how much more fuel will guzzle along to compensate for lesser performance.-

Haven’t had the opportunity to test Inflatable deep V decks nor speed tubes sibs mainly because don’t work with them, both have their own flaws compared to traditional sib/rib own flaws, but I’m sure all of what been written here could be applied to them, a matter to try them out, good luck.-

Happy Boating

 

[jrs_diesel]

Re: ABC Sib-Rib Installation Guidelines.-

Great posts Sea Rider!

Here's a quick video demonstrating my 9 ft SIB with a 2 stroke Johnson 9.9 HP engine. My SIB has an inflatable keel with a wood floor. The transom measured 15 inches from the center of the keel to the top. The 9.9 measured 17? inches from the A/V plate to the transom clamps. So the motor hung down 2? inches too low. In this video, I have 1 inch wood blocks between the clamps and the transom so the motor was only down 1? inches below optimum. Also the tilt pin is stuck in the lowest hole so the motor is trimmed in a little too far.

It doesn't look too bad at first, but around 0:20 in the video when I go full throttle you can see significant splashing from the engine being too low. It splashes the back of the boat, the motor, and a little bit into the boat itself.

 

[Sea Rider]

Re: ABC Sib-Rib Installation Guidelines.-

Hi Jrs_Diesel,

Good to read that you're under the right track for best sib/engine height optimization. Try to shim transom tad more untill splash disappears. Water flow should skim under upper deflector plate, that's at wot and engine sitting at 90? Probably will need to re arrange engine front & read pads heights for engine clamps to be adjusted aroundr middle front pad. Engine will work much better. Is your transom a straight one, or has a lower edge in middle ?

Happy Boating

 

[SIBRider]

Re: ABC Sib-Rib Installation Guidelines.-

Sea Rider you are misguiding readers here. IMHO

For starters. It's controversial topic but at least officially this protrusion on outboard leg is called anti-cavitation plate by every manufacturer (Yamaha, Honda, Suzuki etc). They probably know at least as well as any one of us. Don't you think? See #5.

They are correct of course.

Second topic. When you raise motor you reduce drag, and static water pressure. You make life easier for your motor. It pushes prop harder/faster and you are cavitating. Effectively motor tells you it can do better. I you need to get more aggressive (higher pitch prop) to take advantage of it. Go faster.

 

[SIBRider]

Re: ABC Sib-Rib Installation Guidelines.-

think about it. If you make a car more aerodynamic but don't change anything else meaning you are reaching same engine rpms as before with same gear ratios same tires. Would you go any faster? No! You would use less fuel for sure. To go faster since now that you have less drag you need higher gear ratio. Prop is effectively your gear box (or rather axle/differential since it has constant ratio). Fluid of course complicates it with cavitation. Prop pitch is inverse of angel of attack. Higher angle of attack increases chance of cavitation at given speed.

 

[jrs_diesel]

Re: ABC Sib-Rib Installation Guidelines.-

Hi Jrs_Diesel,

Good to read that you're under the right track for best sib/engine height optimization. Try to shim transom tad more untill splash disappears. Water flow should skim under upper deflector plate, that's at wot and engine sitting at 90? Probably will need to re arrange engine front & read pads heights for engine clamps to be adjusted aroundr middle front pad. Engine will work much better. Is your transom a straight one, or has a lower edge in middle ?

Happy Boating

Since I made that video (July), I built a transom lift to raise the top edge of the transom 2? inches. The A/V plate on the motor sits a ? inch lower than the bottom of the keel now. Should be about perfect. And since my original tilt pin is stuck (thankfully in the lowest hole), I made a few new ones from a steel rod I bought at the hardware store.

I tried to water test it, but the ignition pack on my outboard failed before I even made it out of the no wake zone. I've since fixed the ignition so hopefully I can water test it soon before it gets too cold out.

When I made the transom lift I also marked the top so I can mount the motor and quickly center it. Not sure what you mean by a straight transom or lower edge?

 

[Sea Rider]

Re: ABC Sib-Rib Installation Guidelines.-

SIBRider;

Gee, with just 4 post have become a instant boating expert. Sorry, this is a Tohatsu special in case you haven't read the introduction at all, can't be applied to Yam, Suzuki or other brand engine to start with, all engines have different lower legs configurations.-

It's not anti cavitation plate, it's anti ventilation plate, great difference between both terms, anyway, upper water def plate is not mentioned at all by any OB manufacturer on any owners manual. You need to take a urgent hydrodinamic's lesson, what you have posted as second topic has no rhyme or reason at all.

I'm not going to waist my precious time arguing with theoretical boaters that usually go by owner manual, plain theory doesn't perfectly match real boating world. Last weekend took a Tohatsu factory tecnician for a wot spin, he was astonished to see how a well height seated 18 HP engine could deliver full max revs on a rib that needs a 30 HP minimum. When saying that small deflector plate should be stated along AV plate on owner's manual, he said it should be by it's own right, but manual would have to be rewritten to take this important issue into account.-

Happy Boating

 

[SIBRider]

Re: ABC Sib-Rib Installation Guidelines.-

You got me on number of posts. Hydrodynamics was one bloody complicated subject back at school. Could use a refresher for sure.

Perfect WOT only tells you and/or tech that your motor prop combination is matched nicely to your boat. Doesn't mean you have best possible performance for given boat/motor/prop combination. One could put prop 2 feet under water and have great WOT with lower pitch prop.
I got 300lb SIB 15.5 feet Zodiac F470. Motor raised ~3", "..." plate about 1/2 below transom, upgraded 13" pitch prop, Suzuki 4 Stroke DF25EL (25 hp) motor instead of recommend 40. Gets up to 24-25 mph as it is with me on board.

 

[fbpooler]

Re: ABC Sib-Rib Installation Guidelines.-

Sea Rider put much thought in his extended discussion which is a job well done. I do not understand why anyone would question his conclusions.

There is a difference in air entrained in the water fed to a propeller and cavitation. Cavitation will pit the heck out of prop blades, but entrained water may not depending on the area the air impinges on the blades. The anti cavitation plate is designed to prevent entrained air, therefore anti ventilation plate is a correct description also. Is there a difference between a half dozen and six?

 

[SIBRider]

Re: ABC Sib-Rib Installation Guidelines.-

Sea Rider put much thought in his extended discussion which is a job well done. I do not understand why anyone would question his conclusions.

While SeaRider is correct regarding trim, unfortunately his conclusions about mounting height disagree with multiple conclusions made by other reputable sources, outboard manufacturers notwithstanding. Generally higher mounted motor results in better performance. Paraphrasing "As high as possible as low as necessary" . For example:
Classic Whaler: Boston Whaler: Reference: Performance: Engine Mounting Height
Very fine document: http://media.channelblade.com/boat_...s/dts/dealerlibraries/8441/copetipstricks.pdf
FYI: one of few that does refer to "THE @%&$ PLATE" as anti-ventilation plate ... as well as cavitation plate. Go figure :facepalm:

 

[fbpooler]

Re: ABC Sib-Rib Installation Guidelines.-

I don't believe I have ever seen such weak arguments put forth to counter the work of someone who has taken the time to educate others. The negative comments do not rise to the level of the work done by the OP of this thread which should make mounting an engine an easier operation for the uninitiated. The last time I had contact with someone of this nature was an engineer with OMC who declared that the 85 HP engine on my boat was incapable of performing as it did in actual practice. He had a great theory, but actual experience proved him wrong.

 

[SIBRider]

Re: ABC Sib-Rib Installation Guidelines.-

I don't understand how is evidence supplied by well established industry is week argument. Nor how evident it is that more components you drag through water, more resistance you one would have. Yet my actual personal experience goes against conclusions made by author. That are supposedly gospel for uninitiated.
Here is my boat. F470 Zodiac 15.5 ft long, 192 pounds just a hull throw in rigid floor system at just under 100 lb it's not a slouch. Motor - band new Suzuki 4 DF25EL - 4 Stroke, long shaft. Nice but considerably smaller than recommended 40 HP.
As I put boat together, motor setup incidentally corresponded to that of the author advice: Deflector plate nearly even/or slightly above with the keel. As evident here:

Boat performance was ok. Maxing out at 20 mph we me on board and about 17 with my family of 3 (2 adults, 3 kids).
Well aware of industry advice to mount motor with cavitation plate within 1" range below transom I built 3" riser for my motor.
Some AL plate with HDPE block sandwiched in between.

Now my anti-cavitation plate is just under transom level as evident here.

Now my engine was able to spin much freer I was able to upgrade my prop and enjoy much improved performance. With passenger load of roughly 700lb (3 adults and 3 kids) it gets up to about 20-21, and just by myself I got over 25mph. What's not to like? Yet if I adhered to author's advice I would not be there, naturally.