ABC Sib-Rib Installation Guidelines.-

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A Tohatsu Especial.-

Decided to write & post these handy 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 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.-

In order to achieve best boat plane and engine top performance you need to have 3 issues very clearly defined and understood which are Sib-Rib Installation, Transom Height and 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 were factory transom height modified to state of the art match Tohatsu’s lower units.-

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.-

To fully understand these guidelines tech concepts, will be divided into 5 main topics which are : (1) Boat Balance & Trim (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 low HP OB’s or unhealthy engine & prop not delivering full top rpm.-

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

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(1) Boat Balance & Trim : Float sib/rib alone completely empty with fuel tank the furthest 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 angle so thrust rod position could vary accordingly 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 on that position on all boating water conditions and load, remember is a fixed manual trim OB. Tight engine clamps for rear of swivel bracket to remain parallel and resting flat against transom’s rear plate. Some ribs have a upper transom edge border that needs to be filled for swivel bracket to remain at perfect alignment angle rest against rear plate.-

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

Water Test : Before launching, with gauge inflate sib 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 performance as ribs rides on its hull not on tubes.-

On flat calm glassy, no wind waters, sit boater mate up front towards bow to maintain sib/rib at natural floating position. Tiller driver & spotter must sit inside deck next to transom. Full throttle, check if achieving transom water splashes to know which part of lower leg is producing it and if in need to modify transom height accordingly.

Sorry folks, the only way to state of the art check water back splash issues is pulling driver/spotter head out transom (4). For boating perfection, it’s not I believe, I assume have a perfect match, I’ll ask at the forums, it’s one time visual check and afterwards a on land correction if applicable.-

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

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(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 upper round or square borders (red X) and a lower water attach edge (AE) (Green X) Water splash types :

Out transom splash (1) is usually produced when Water Flow hits attack edge at speed, water swirls and climbs through 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 tech bummer.-

Over transom splash, is usually produced when WF at speed hits leg’s round (2) or square (3) upper border located above WDP. When sib-rib is under way at medium to wot, will produce worst horrible back transom splashes along excessive tail drag & deck flooding. Noticeable when engine sits too low on transom than needed.-

Boater can’t go boating at all throttle range 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 (4) won’t see 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 be achieved.-

If having out or over transom unwanted water splashes is entirely up to boater if confortable riding under or over trimmed or willing to chop down, or rise transom height accordingly. Modifying transom’s height it’s time demanding, costly if doing at a boat repair shop along precious wasted time under trail & error til correct engine height is matched.-

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

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(3) Engine/Transom Height Match : Tohatsu, Merc, Nissan are born being natural over transom water splashers compared to other brands like Yamaha & Suzuki mostly due to having a built square border just above water deflector plate in comparison to rounded borders found in other brands.

The usual forum feed back talk is recommending 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 as there are too many tech variables to take into account : hull shapes, lengths, widths, tube’s diam, transom height all which can vary from sib to sib, rib to rib as all are not made factory same. Same engine will not work appropriate on all same brand sib/rib models.-

Personally 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 trial & error water testing.-

Once you have the sweet height spot correctly dialed in, this set up gives excellent hole shot, immaculate close turns at speed, betters engine’s top rpm & prop thrust optimizing sib/rib performances under all water conditions.- Had a great time water testing same Tohatsu engine sitting at 90° at different transom heights and this is what was discovered during sea trail.-

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(4) Trim Angle : A prop works on an engine 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 at plane.-

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 under or over trimmed will cause inevitable AV drag and thrust loss, noticeable when achieving low or high rear middle wake when on plane.-

Related Links :

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

Silverado 40S & Tohatsu 18 - YouTube

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5-Sib-Rib Engine Optimization : Once engine is working optimum, you could play with engine prop to deliver top rpm to optimize your intended type of boating. Will need to install and 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 min-max wot safe 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, your combo will plane much faster on next outing.-

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(6) Special Configurations : Some low overboard rescue ribs, surfer transports, large tenders are being factory delivered with just 18” transom heights rather than 20" transoms.-

-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 or add a second water deflector plate (3) above lower one to divert back splash against water flow. Entirely up to you, your own decision.-

For ending this extensive post, must say that 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 has been posted here could be applied to their paricular cases, a matter to try them out, good luck.-

Happy Boating